smoke on test drive
11-15-2012, 11:08 PM
#16
Mmm... I know someone from my home town that own's a mint 996 tt which has covered 48,000 miles now. I looked at his for reference before buying my own and he told me then one of the characteristics of his car was to bellow white exhaust gases on start up but not just from cold! He said he noticed it sometimes if he left it sitting for a short time after running.
He has had the car serviced at OPC throughout it's life and this has never been addressed by them. I distinctly remember him at the time telling me how embarrasing it was when it happened. He definitely thought it was intrinsic to the Metzer engine. His car runs perfect??
Rob
He has had the car serviced at OPC throughout it's life and this has never been addressed by them. I distinctly remember him at the time telling me how embarrasing it was when it happened. He definitely thought it was intrinsic to the Metzer engine. His car runs perfect??
Rob
Can't say i'd be happy if mine did that everytime i started it. It park it back in their dealership quick smart.
Its been so long since mine's done it i cant even recall the last time it was it was so long ago now. But i do know there will be possibly be a time in the future when i least expect it that it will happen again. With Summer just around the corner maybe i'm bound to get an occurrance then, but i always idle my car for 20 to 30 seconds before switch off anyway, particularly after a bit of spirited driving. It allows the engine temperatures to normalise, and oil viscosity to recover, and the turbo speeds to neutralise plus you're less likely to get any residual fuel land in any cylinder from an active injector that's just let a squirt of fuel go at the exact time of ignition kill...left uncombusted, sitting in the cylinder. Shutting the engine off straight away is not the best way to go if you can avoid it.
Fwiw, my take is it's more fuel related than oil related purely because of the colour of the smoke.
11-16-2012, 12:51 AM
#17
Race Director
The Turbo engine doesn't have the infamous M96 engine AOS but it has an AOS nonetheless.
It is part of the oil tank, its baffling. I have been told by techs who have had access to a cutaway of this tank that it is pretty sophisiticated inside.
One reason why overfilling the engine oil tank is bad is the higher oil level I've been told reduces the efficiency/effectiveness of the oil tank AOS feature.
The smoking all the time -- so to speak -- is abnormal behavior. While these engines smoke once in a while one that smokes every startup is rare. I have not encountered but just IIRC one report of one of these engines doing this, and it is yours.
My concern is that the smoking is a turbo seal problem or possibly one or more bad valve stem seals or excessive clearance at the valve stem/guide interface.
Yet another source is a cylinder/piston/ring interface.
Regardless I think you can find a better example.
It is part of the oil tank, its baffling. I have been told by techs who have had access to a cutaway of this tank that it is pretty sophisiticated inside.
One reason why overfilling the engine oil tank is bad is the higher oil level I've been told reduces the efficiency/effectiveness of the oil tank AOS feature.
The smoking all the time -- so to speak -- is abnormal behavior. While these engines smoke once in a while one that smokes every startup is rare. I have not encountered but just IIRC one report of one of these engines doing this, and it is yours.
My concern is that the smoking is a turbo seal problem or possibly one or more bad valve stem seals or excessive clearance at the valve stem/guide interface.
Yet another source is a cylinder/piston/ring interface.
Regardless I think you can find a better example.
11-16-2012, 01:26 AM
#18
The Turbo engine doesn't have the infamous M96 engine AOS but it has an AOS nonetheless.
It is part of the oil tank, its baffling. I have been told by techs who have had access to a cutaway of this tank that it is pretty sophisiticated inside.
One reason why overfilling the engine oil tank is bad is the higher oil level I've been told reduces the efficiency/effectiveness of the oil tank AOS feature.
The smoking all the time -- so to speak -- is abnormal behavior. While these engines smoke once in a while one that smokes every startup is rare. I have not encountered but just IIRC one report of one of these engines doing this, and it is yours.
My concern is that the smoking is a turbo seal problem or possibly one or more bad valve stem seals or excessive clearance at the valve stem/guide interface.
Yet another source is a cylinder/piston/ring interface.
Regardless I think you can find a better example.
It is part of the oil tank, its baffling. I have been told by techs who have had access to a cutaway of this tank that it is pretty sophisiticated inside.
One reason why overfilling the engine oil tank is bad is the higher oil level I've been told reduces the efficiency/effectiveness of the oil tank AOS feature.
The smoking all the time -- so to speak -- is abnormal behavior. While these engines smoke once in a while one that smokes every startup is rare. I have not encountered but just IIRC one report of one of these engines doing this, and it is yours.
My concern is that the smoking is a turbo seal problem or possibly one or more bad valve stem seals or excessive clearance at the valve stem/guide interface.
Yet another source is a cylinder/piston/ring interface.
Regardless I think you can find a better example.
Same if there was a fault with the oil separator. You would have the problem all the time. This smoke problem is intermitant though so logically those issues need to be removed from the equation as being a cause.
11-16-2012, 02:14 AM
#19
Ok. heres more food for thought guys.
The engine is dry sumped although there is an area in the base (oil pan) to collect the oil as it runs off and away from all the pressure fed areas so to be returned back to the primary tank via the pick up.
So it's not as if the horizontally opposed cylinders are ever left swimming or drowning in a sea of oil at any stage for the oil to pass back past the rings into the combustion chamber.
Now unless the engine has been overfilled where the primary tank is full along with the oil pan, there isn't the environment for oil to flow back up past the rings after shutdown. Now we all know that overfilling can cause engine failure from the hydraulic actions of the oil thrashing/resisting against the pistons during their individual movements. And yes in that instance oil can pass the rings into the combustion chambers for sure......causing further hydraulic locking.
So let's say even if some oil did splash onto the liner walls from the lubrication bleeding off the reciprocating crank or, oil left on the liner walls from the oil squirters during the engine running, or after (say) being stopped suddenly and not fully swept away. However, IF any of that oil is left sat on the liner walls after the engine stopped then no big deal because that oil is always outside of the combustion chambers anyway. And, there wouldn't be enough of it to ever run past the rings. So when the engine next fires up it's not going to cause any oil smoke because any of that oil is simply sitting on the wrong side of the combustion chamber to ever be part of any ignition/combustioncycle anyway. So, this oil serves only to lubricate the rings on the downward and then upward stroke, (which is the case) as what happens during normal engine operation as the oil is delivered to that area. Sure there is always a tiny amount of oil that gets burned along with the fuel on each combustion stroke (called oil consumption....yes every engine does use oil lol), as what has to happen as a consequence of it lubricating the liner walls from the each passing stroke of the piston and rings, but, in saying that remember the rings will always do the job of removing any excess quantity of oil from entering the combustion chamber that is being delivered to that area by the oil squirters and reciprocating crank etc on the downward stroke....pushing it back into the oil pan for collection.
Ok. Now remember the AOS is there primarily to stop/spearate any oil from the vapours in the crankcase from recirculating into the intake. So oil should not be passing into the intake. And if it is there should be smoke either all the time and/or at every start up.
So how is oil getting into the cylinders after shutdown? It cant be. As i said earlier. The most logical reason is fuel residue left sitting idle in the cylinder after shutdown. So, the engine fires up, new fuel is delivered into the cylinders and in that process joins along with any residual fuel left in the (offending) combustion chamber, which of course then becomes all too much to properly combust in the initial firing cycle (AFR). That unburned fuel smoke then exits the cylinder via the exhaust port/s and then passes through the catalitic converter, which are not yet at sufficient/correct operating temperature to arrest the unburnt fuel/emmissions, and then that smoke exits the tailpipe, whitish grey in colour.
That's my take anyway. Fwiw.
The engine is dry sumped although there is an area in the base (oil pan) to collect the oil as it runs off and away from all the pressure fed areas so to be returned back to the primary tank via the pick up.
So it's not as if the horizontally opposed cylinders are ever left swimming or drowning in a sea of oil at any stage for the oil to pass back past the rings into the combustion chamber.
Now unless the engine has been overfilled where the primary tank is full along with the oil pan, there isn't the environment for oil to flow back up past the rings after shutdown. Now we all know that overfilling can cause engine failure from the hydraulic actions of the oil thrashing/resisting against the pistons during their individual movements. And yes in that instance oil can pass the rings into the combustion chambers for sure......causing further hydraulic locking.
So let's say even if some oil did splash onto the liner walls from the lubrication bleeding off the reciprocating crank or, oil left on the liner walls from the oil squirters during the engine running, or after (say) being stopped suddenly and not fully swept away. However, IF any of that oil is left sat on the liner walls after the engine stopped then no big deal because that oil is always outside of the combustion chambers anyway. And, there wouldn't be enough of it to ever run past the rings. So when the engine next fires up it's not going to cause any oil smoke because any of that oil is simply sitting on the wrong side of the combustion chamber to ever be part of any ignition/combustioncycle anyway. So, this oil serves only to lubricate the rings on the downward and then upward stroke, (which is the case) as what happens during normal engine operation as the oil is delivered to that area. Sure there is always a tiny amount of oil that gets burned along with the fuel on each combustion stroke (called oil consumption....yes every engine does use oil lol), as what has to happen as a consequence of it lubricating the liner walls from the each passing stroke of the piston and rings, but, in saying that remember the rings will always do the job of removing any excess quantity of oil from entering the combustion chamber that is being delivered to that area by the oil squirters and reciprocating crank etc on the downward stroke....pushing it back into the oil pan for collection.
Ok. Now remember the AOS is there primarily to stop/spearate any oil from the vapours in the crankcase from recirculating into the intake. So oil should not be passing into the intake. And if it is there should be smoke either all the time and/or at every start up.
So how is oil getting into the cylinders after shutdown? It cant be. As i said earlier. The most logical reason is fuel residue left sitting idle in the cylinder after shutdown. So, the engine fires up, new fuel is delivered into the cylinders and in that process joins along with any residual fuel left in the (offending) combustion chamber, which of course then becomes all too much to properly combust in the initial firing cycle (AFR). That unburned fuel smoke then exits the cylinder via the exhaust port/s and then passes through the catalitic converter, which are not yet at sufficient/correct operating temperature to arrest the unburnt fuel/emmissions, and then that smoke exits the tailpipe, whitish grey in colour.
That's my take anyway. Fwiw.
Last edited by speed21; 11-16-2012 at 02:50 AM.
11-16-2012, 12:37 PM
#20
Race Director
Weak or worn/faulty valve stem seals or excess valve to guide clearnces would exibit blue smoke exiting the tailpipes on the overun though. Same with excessive piston to bore clearnces, worn rings or insufficient tangential load from the rings allowing oil to pass. You would have signs of oil smoke under power/load, and also excessive oil consumption along with the smoke all the time.
Same if there was a fault with the oil separator. You would have the problem all the time. This smoke problem is intermitant though so logically those issues need to be removed from the equation as being a cause.
Same if there was a fault with the oil separator. You would have the problem all the time. This smoke problem is intermitant though so logically those issues need to be removed from the equation as being a cause.
As for the other possible explanations again it depends upon the severity of the condition.
The engine can't be too sick since the CEL is not on. I wonder what the emissions readings would be though?
The OP reported he ran the engine up and it smoked. Was this smoking as the rpms increased (rings and associated hardware or turbo seals), or when the engine was on closed throttle (valve guide/seals or again turbo seals)?
Just because the OP reported no smoking under hard acceleration or under closed throttle coast down is not conclusive this is not going on. He may not have been that observant. And under some conditions this smoking is hard to see. I like to use the low morning or evening sunlight and drive away from the direction of the sunlight to let it provide a backlight. Then I accelerate hard up through as many gears as I can while checking for smoking. Then I coast the car to a slower speed under closed throttle then at some point nail the gas pedal. If I see no smoking or what I see is not oil related (it can sometimes be fuel related though in modern engines with their computer controller fueling this is no longer the case) then the car passes this check.
As for oil consumption unless the oil consumption is very very bad no way even a 30 or 1 hour test ride/drive of a car will turn up an oil consumption problem.
Regardless of why the engine is smoking, to me is makes no difference. The engine is smoking at abnormal times. This is sufficent reason in my opinion to reject the car.
11-16-2012, 01:19 PM
#21
Unfortunately I think your right again speed! I agree absolutely. My understanding concurs with your explanation in relation to the scraper ring sweeping the bore. If these rings were not sealing effectively - especially on a tt with pressurised delivery/ strategic oil jets & horizontal cylinders - when the car was running it would be burning as much oil as petrol with two blue chimney stack's out the back.
I think it's fuel aswell. The colour of the residue and the fact I know as said in an earliier thread of a 996tt locally that smokes heavily sporadically and yet has covered 48k mls runnig perfectly logically suggests there are no intrinsic mechanical flaws in the engine.
Maybe Kevin or one of the tuners could advise further.
Rob.
I think it's fuel aswell. The colour of the residue and the fact I know as said in an earliier thread of a 996tt locally that smokes heavily sporadically and yet has covered 48k mls runnig perfectly logically suggests there are no intrinsic mechanical flaws in the engine.
Maybe Kevin or one of the tuners could advise further.
Rob.
11-16-2012, 03:03 PM
#22
Race Director
Well the AOS in whatever guise can and I believe account for some of the smoking, the normal smoking.
It happens thus: The AOS fails to remove all the oil vapor. This vapor is routed into the intake manifold and that last sharp turn has the oil droplets impacting the intake wall surfaces. Techs I speak too tell me it is quite common to whenever access to this area is available to find the intake inner walls have oil on them. sometimes wet with oil.
The hot engine is shut off. The oil on the intake walls helped by the heat of the engine heat soaking into everything runs down and into one or more cylinders.
Next time the engine is started, there is oil smoke.
Fuel smoke is different and the engine generally reacts to this if there's enough fuel to cause startup smoke. Besides the air is thick with the smell of unburned gasoline.
Besides when the engine is first cranked at start time the injectors are not triggered, spark is not triggered until the engine is spinning at some speed. This takes a few revs for the engine is only cranked to around 100 rpms is the best info I have.
Whatever unburned fuel there is from the less than ideal combustion at this time can certainly contribute to visible exhaust smoke and smell.
When the engine is shut off the injectors are not triggered as the engine spins to a halt.
Now there is the possibility one or more injectors are leaking. But again this is going to create a different flavor/odor of start up smoke and the engine will blubber or race a bit then settle down. Furthermore if the injector is leaking at engine off it will leak when the engine is running and the bank of that cylinder will run rich and the DME will adapt/compensate and lean all cylinders out on that bank. This is reflected in short/long term fuel trims of that bank and compared to the other bank they can be quite different.
If the leaking is bad enough the leaning is enough the good cylinders will misfire.
It happens thus: The AOS fails to remove all the oil vapor. This vapor is routed into the intake manifold and that last sharp turn has the oil droplets impacting the intake wall surfaces. Techs I speak too tell me it is quite common to whenever access to this area is available to find the intake inner walls have oil on them. sometimes wet with oil.
The hot engine is shut off. The oil on the intake walls helped by the heat of the engine heat soaking into everything runs down and into one or more cylinders.
Next time the engine is started, there is oil smoke.
Fuel smoke is different and the engine generally reacts to this if there's enough fuel to cause startup smoke. Besides the air is thick with the smell of unburned gasoline.
Besides when the engine is first cranked at start time the injectors are not triggered, spark is not triggered until the engine is spinning at some speed. This takes a few revs for the engine is only cranked to around 100 rpms is the best info I have.
Whatever unburned fuel there is from the less than ideal combustion at this time can certainly contribute to visible exhaust smoke and smell.
When the engine is shut off the injectors are not triggered as the engine spins to a halt.
Now there is the possibility one or more injectors are leaking. But again this is going to create a different flavor/odor of start up smoke and the engine will blubber or race a bit then settle down. Furthermore if the injector is leaking at engine off it will leak when the engine is running and the bank of that cylinder will run rich and the DME will adapt/compensate and lean all cylinders out on that bank. This is reflected in short/long term fuel trims of that bank and compared to the other bank they can be quite different.
If the leaking is bad enough the leaning is enough the good cylinders will misfire.
11-16-2012, 04:03 PM
#23
You know what macster, I think your the man with his finger on the pulse here!
Speed tends to gets lost any further in than the exhaust
Seriously, plausable theory! Not sure who to back now!
R
Speed tends to gets lost any further in than the exhaust
Seriously, plausable theory! Not sure who to back now!
R
11-16-2012, 04:17 PM
#24
Rennlist Member
Thread Starter
Today I drove another one (gray/tan) and it was exactly what I expected. The power was smooth and the pulled clean after it built boost. There was no smoke except the logical bit from startup (which I asked the dealer to not start it until I got there). The oil meter was right and the pressure gauges wasn't pinned to the top like the silver/black one earlier. Man that torque is crazy fun and I'm driving a torque monster E55 wagon.
11-16-2012, 05:38 PM
#25
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If the car in question rose to the top of your buying list. I would instruct the tech to pull the pressure hose between each turbocharger and intercooler. A failing turbocharger will produce oil that will accumulate in the compressor housing and fill the pressure hose. More oil will show up sooner/quicker in the low pressure plastic inlet pipes, but it's easier and quicker to check (labor) the rubber pressure hoses.
11-16-2012, 08:04 PM
#27
Drifting
Today I drove another one (gray/tan) and it was exactly what I expected. The power was smooth and the pulled clean after it built boost. There was no smoke except the logical bit from startup (which I asked the dealer to not start it until I got there). The oil meter was right and the pressure gauges wasn't pinned to the top like the silver/black one earlier. Man that torque is crazy fun and I'm driving a torque monster E55 wagon.
You will find a car.. Just be patient and get EXACTLY what you're looking for. She is out there somewhere..
11-16-2012, 09:33 PM
#28
[quote=Macster;10003978]
Not where it is functioning correctly. The oil is being adequately separated from the vapours or there would be other signs and symptoms.
Inside every intake manifold you will find a sticky light film which is a concoction of vapours drawn in via the crankcase. If there were oil droplets entering you would have a smokey engine all the time along with excess oil consumption. It doesn't take much oil at all to create blue smoke out the tailpipe either which is a good tell tale sign if that is occurring.
It depends on what one refers to as being "wet" with oil. An inlet manifold or plenum chamber that has area of obvious oil liquid build up indicts a problem with either a malfunctioning separator or worn/faulty oil rings. But again you will have visible smoke under hard throttle as that liquid is ingested under high intake vacuum.
This can happen if there is a separator problem but again this symptom is generally joined with excessive oil consumption and other symptoms as the separator is allowing the oil to pass during engine normal operation as well. You can't have one without the other.
Incorrect. It is generally assumed that unburned fuel is associated to only a black smoke, not a white/grey smoke, which is sometimes incorrectly though as being oil smoke. However, unburned fuel from an over fuelling condition is generally only black under power/load operating conditions. On cold start the engine has its own in built functions (which the porsche engine has ie cold start function etc) to get the engine going quickly. There are a range of sensors that are all trying to register in a microsecond and relay that info to the ecu. Often battery power is not at its optimum (engine bay cooling fans running after shutdown), often sensors have a slight fouling or are not precisely within correct assembly position (refer crank angle sensor issue which can cause over rev on start up etc....another story altogether) but essentially the start up settings are different to the settings which return once the engine is at operating temperature or, the catalitic converter temps have been deemed satisfied.
No that is not correct. An engine does not have to be spinning at "some speed" for that to occur. The moment the key is turned and power is delivered to the starter motor (on its first movement) along with all the sensors, all these things (injectors, spark etc) are all operational. As i mentioned above it is only if the battery power is not at optimum power that things can momentarily alter for that micro second but any discrepancy will resolve itself once full power to the sensors is restored.....which is within a micro second. Thats why an engine will generally crack into life immediately without the need to be wound over and over on the starter.
One that requires constant winding beyond the first rotation of the crank indicates the sensors are still unable to register properly for the ECU to respond.
Correct, which is why any residual fuel left over will upset the air to fuel ratio (AFR) in an already rich idle environment provided initially by the cold start function along with different variocam positions, throttle position and so on etc..which all have their parts to play during warm up phase.
It's all a fine balance at that point, so just the slightest thing out can create an issue for that split second....ie such as that whitish grey smoke occurrence.
This is an area which requires more depth of discussion for me to explain my own take here but suffice to say most of it really isnt relevant in an engine with such a sporadic smoke problem.
Petrol engine injectors are a solenoid type which have them deactivate the moment ignition is cut so there really isn't the opportunity for any of them to leak after shutdown. The fact the problem can occur on a new engine or near new engine indicates it wouldn't be from worn injector seats. I would however agree there is still high line pressure present between the high pressure fuel pump and the injectors, which incidentally are not directly mounted into the combustion chambers as DFI injectors are. Therefore there is the possibility that during that split second the ignition is cut one of the injectors has already unloaded its measure into the manifold where it can then run onto the combustiion chamber un burned ...as what i feel could be a likely culprit. Bring into the equation a high ambient temperature environment in the engine bay after a harder than normal run and on a hotter than normal day and that ambient temperature rises even more again, placing more load with the expansion pressures. Fuel under pressure in the line can have a tendency to expand and push against the sealing of the seats of the injectors yet by rights should still be retained if the condition is good. Its amazing what can happen in practice which defies the theory. But these are unknown factors so it is possible a slight fuel leakage could occur. It's a hard call.
Where you previously mention leaky turbo seals etc as allowing oil to build up into the manifold and then run down after shutdown again what you have to contened with that theory is that this sporadic smoke on start up issue also happens on the naturaaly aspirated Porsche engines. Ive seen it common on GT3 engine's for e.g..which are another highly tuned/fuel engine.
The other thing is if you have a leaky turbo shaft seal you will have other ways of chasing that fault down by removing the rubber hose off the turbo or intercooler. I don't see this as being associated to the start up smoke problem though. I have also heard of instances where those hoses have been found to contain a large pool of oil and it is only under boost that the oil then gets drawn or blown into the intake causing excessive oil smoke out the pipes. But again the intercoolers would be pretty soaked as well and would need flushing etc along with all the tubing and so forth. Theres a bit involved in addressing that issue.
Again this is just my take on the sporadic smoke issue and for all I know Porsche already knows the precise reason why this is occurring but is happy to leave it as a mystery for the enquirer on the basis the issue is occurring due to an inherent design with the fuel system or engine management system etc.
Note" I am also not sure if this sporadic white smoke problem is occurring on the DFI engine where the injectors deliver directly inside the combustion chamber.
It would be interesting to get feedback from owners of these cars.
I will say if an engine does it every time it starts up or, during drive time i wouldn't buy that car either.
Well the AOS in whatever guise can and I believe account for some of the smoking, the normal smoking.
It happens thus: The AOS fails to remove all the oil vapor. This vapor is routed into the intake manifold and that last sharp turn has the oil droplets impacting the intake wall surfaces.
Techs I speak too tell me it is quite common to whenever access to this area is available to find the intake inner walls have oil on them. sometimes wet with oil.
The hot engine is shut off. The oil on the intake walls helped by the heat of the engine heat soaking into everything runs down and into one or more cylinders.
Next time the engine is started, there is oil smoke.
Next time the engine is started, there is oil smoke.
Fuel smoke is different and the engine generally reacts to this if there's enough fuel to cause startup smoke. Besides the air is thick with the smell of unburned gasoline.
Besides when the engine is first cranked at start time the injectors are not triggered, spark is not triggered until the engine is spinning at some speed. This takes a few revs for the engine is only cranked to around 100 rpms is the best info I have.
One that requires constant winding beyond the first rotation of the crank indicates the sensors are still unable to register properly for the ECU to respond.
Whatever unburned fuel there is from the less than ideal combustion at this time can certainly contribute to visible exhaust smoke and smell.
It's all a fine balance at that point, so just the slightest thing out can create an issue for that split second....ie such as that whitish grey smoke occurrence.
When the engine is shut off the injectors are not triggered as the engine spins to a halt.
Now there is the possibility one or more injectors are leaking. But again this is going to create a different flavor/odor of start up smoke and the engine will blubber or race a bit then settle down. Furthermore if the injector is leaking at engine off it will leak when the engine is running and the bank of that cylinder will run rich and the DME will adapt/compensate and lean all cylinders out on that bank. This is reflected in short/long term fuel trims of that bank and compared to the other bank they can be quite different.
If the leaking is bad enough the leaning is enough the good cylinders will misfire.
Now there is the possibility one or more injectors are leaking. But again this is going to create a different flavor/odor of start up smoke and the engine will blubber or race a bit then settle down. Furthermore if the injector is leaking at engine off it will leak when the engine is running and the bank of that cylinder will run rich and the DME will adapt/compensate and lean all cylinders out on that bank. This is reflected in short/long term fuel trims of that bank and compared to the other bank they can be quite different.
If the leaking is bad enough the leaning is enough the good cylinders will misfire.
Petrol engine injectors are a solenoid type which have them deactivate the moment ignition is cut so there really isn't the opportunity for any of them to leak after shutdown. The fact the problem can occur on a new engine or near new engine indicates it wouldn't be from worn injector seats. I would however agree there is still high line pressure present between the high pressure fuel pump and the injectors, which incidentally are not directly mounted into the combustion chambers as DFI injectors are. Therefore there is the possibility that during that split second the ignition is cut one of the injectors has already unloaded its measure into the manifold where it can then run onto the combustiion chamber un burned ...as what i feel could be a likely culprit. Bring into the equation a high ambient temperature environment in the engine bay after a harder than normal run and on a hotter than normal day and that ambient temperature rises even more again, placing more load with the expansion pressures. Fuel under pressure in the line can have a tendency to expand and push against the sealing of the seats of the injectors yet by rights should still be retained if the condition is good. Its amazing what can happen in practice which defies the theory. But these are unknown factors so it is possible a slight fuel leakage could occur. It's a hard call.
Where you previously mention leaky turbo seals etc as allowing oil to build up into the manifold and then run down after shutdown again what you have to contened with that theory is that this sporadic smoke on start up issue also happens on the naturaaly aspirated Porsche engines. Ive seen it common on GT3 engine's for e.g..which are another highly tuned/fuel engine.
The other thing is if you have a leaky turbo shaft seal you will have other ways of chasing that fault down by removing the rubber hose off the turbo or intercooler. I don't see this as being associated to the start up smoke problem though. I have also heard of instances where those hoses have been found to contain a large pool of oil and it is only under boost that the oil then gets drawn or blown into the intake causing excessive oil smoke out the pipes. But again the intercoolers would be pretty soaked as well and would need flushing etc along with all the tubing and so forth. Theres a bit involved in addressing that issue.
Again this is just my take on the sporadic smoke issue and for all I know Porsche already knows the precise reason why this is occurring but is happy to leave it as a mystery for the enquirer on the basis the issue is occurring due to an inherent design with the fuel system or engine management system etc.
Note" I am also not sure if this sporadic white smoke problem is occurring on the DFI engine where the injectors deliver directly inside the combustion chamber.
It would be interesting to get feedback from owners of these cars.
I will say if an engine does it every time it starts up or, during drive time i wouldn't buy that car either.
11-17-2012, 04:56 AM
#29
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While the stock 997TT has the KO4 bearing housing and uses one single bearing/bushing. It does share the K16 and K24 sealing rings on the turbine side. Note that the compressor side sealing ring is smaller in diameter (compared to the K16's and K24) Hot shutdowns will increase the coke buildup on the hot side (turbine side). I have already seen accelerated wear to the turbine side seal bore/and widening of the sealing ring groove on many 997.1TT As the gaps wear and grow, combustion exhaust gases from the exhaust side will breach the turbine side seal (from the wear). The exhaust gas pressure is greater vs the engine oil pressure that is inside the bearing housing. The oil passes thru the compressor side sealing ring and enters the charge air intake air. The oil that was once intended to lubricate the turbocharger bearings is now in the Intercooler and ingested into the engine. When you shut the engine down the turbine wheels are still spinning, the bearing housing oil pressure is now greater than the exhaust gas pressure> and the cycle is now reversed. Oil migrates into the turbine housing (Very small amount). When you start the engine the oil gradually burns off > the turbine housing, turbine wheel and VNT blades. Infact most of the oil is burnt in the cats. It is not a complete burn, more of a smoldering smoke show (wet newpaper on a new fire)..
The oil pooling in the comprssor housing or charge air hoses is done on shut down. Depending on the amount of oil determines the smoke show. This condition can occur during acceleration with boost. It is not a exact science. We are trying to pinpoint the oil, I wrote earlier pulling the charge air hoses will tell you how bad your turbochargers are failing. Over filled engines will exhibit the same condition> the main oil tank breather is routed into the passenger side intake/suction ducting for the turbocharger. Since the intake charge air is merged at the "Y" pipe oil is burnt on both engine banks and out each turbocharger/tailpipe. If the intake hoses and intercoolers are clean then you have engine piston ring wear or valve guide wear. A engine leakdown test will tell you which component is wearing/culprit.
On our engines the number 2 piston ring is pinned (it won't rotate around the piston). When the engine are new, the rings must seat. Prior to seating the sealing/piston rings will rotate (number two can't) If the top compression ring rotates to the 6 o'clock position you have a higher risk of oil migration from the engine to the combustion chamber. If it lines up with the pinned number 2 ring> you get ring stack. Oil will pass thru these two gaps freely (RARE condition) It is the luck of the draw, some engines will smoke more than others. If one piston is on Bottom Center in the stroke more oil will have a chance to migrate into the combustion chamber. I would say that every flat 6 engine will have some sort of start up smoke show when left to sit in the garage. Some never do but then some do.
The 993TT have severe oil migration issue. I have manufactured Tungsten turbine sealing rings that are stepped (like a dove tailed wood joint) gapped. Once they seat it is very difficult for exhaust pressure or oil to breach the mechanical stepped seal. All of my 996TT and 997TT turbocharger repairs or builds receive this new turbine seal.
Lastly, when you guys change your oil, pull the charge air hose off the turbocharger compressor housing (remove hose clamp). Check for oil. If the hose has a light film of oil, this is normal. Oil dripping out of the hose points to a failing turbine seal!
The oil pooling in the comprssor housing or charge air hoses is done on shut down. Depending on the amount of oil determines the smoke show. This condition can occur during acceleration with boost. It is not a exact science. We are trying to pinpoint the oil, I wrote earlier pulling the charge air hoses will tell you how bad your turbochargers are failing. Over filled engines will exhibit the same condition> the main oil tank breather is routed into the passenger side intake/suction ducting for the turbocharger. Since the intake charge air is merged at the "Y" pipe oil is burnt on both engine banks and out each turbocharger/tailpipe. If the intake hoses and intercoolers are clean then you have engine piston ring wear or valve guide wear. A engine leakdown test will tell you which component is wearing/culprit.
On our engines the number 2 piston ring is pinned (it won't rotate around the piston). When the engine are new, the rings must seat. Prior to seating the sealing/piston rings will rotate (number two can't) If the top compression ring rotates to the 6 o'clock position you have a higher risk of oil migration from the engine to the combustion chamber. If it lines up with the pinned number 2 ring> you get ring stack. Oil will pass thru these two gaps freely (RARE condition) It is the luck of the draw, some engines will smoke more than others. If one piston is on Bottom Center in the stroke more oil will have a chance to migrate into the combustion chamber. I would say that every flat 6 engine will have some sort of start up smoke show when left to sit in the garage. Some never do but then some do.
The 993TT have severe oil migration issue. I have manufactured Tungsten turbine sealing rings that are stepped (like a dove tailed wood joint) gapped. Once they seat it is very difficult for exhaust pressure or oil to breach the mechanical stepped seal. All of my 996TT and 997TT turbocharger repairs or builds receive this new turbine seal.
Lastly, when you guys change your oil, pull the charge air hose off the turbocharger compressor housing (remove hose clamp). Check for oil. If the hose has a light film of oil, this is normal. Oil dripping out of the hose points to a failing turbine seal!
11-17-2012, 06:20 AM
#30
Kevin i dont disagree with the wear processes associated to turbo seal leakage which (can) occur in these engines or any turbo engine for that matter (and the symptoms that usually follow) even though these types of abnormal wear scenarios generally fit engines which have seen either higher mileage or poor shutdown procedures, mainly after hot running. You wouldn't expect to find this kind of problem in an engine that has only done 2 or 3000kms from brand new unless it had been subjected to extreme abuse or, had faulty turbo chargers from new.
However, this particular intermittent smoke symptom commonly reported by owners is occurring also on naturally aspirated engines too. Therefore, logically the turbo seals must be ruled out in this particular instance. Also the problem is not limited to high mileage engines.
I recall when my own engine demonstrated the problem the first time (near new) i had the turbos inspected under warranty and all was found in to be in perfect working order/condition. No oil residue in the turbo hoses etc. The other point worth mentioning is often this smoke comes out from only one tailpipe and that can sometimes alternate. So it brings to mind the other possible causes for this type of symptom.
As you are aware the catalytic converters will arrest the emissions once at operating temperature with exception to the usual over fuelling type scenario which tends to happen in turbocharged engines during initial WOT where you will commonly see black smoke being emitted from the pipes as the engine speed/load catches up to the fuel feed, depending upon tune settings and other environmental factors of course. This is fairly standard behaviour for any turbocharged engine as i'm sure you are aware. I have yet to see one that doesn't puff a degree of black smoke out the pipes when suddenly demanded WOT. So again it comes back down to evaluating and analysing the most logical/likely causes after going through the routine processes of elimination in diagnosing the actual cause of the symptom in question.
At this stage, after discussing this particular symptom with two close industry associates expert in the field of product failure and engine diagnosis we are to the general consensus it is a residual fuel deposit issue or an inherent design issue associated to the fuel system more than anything else. So nothing to get too alarmed about we feel.
Without evidence of wear in the turbochargers or having obvious oil deposits found in the intake plenum that's the most logical answer at this stage.
So, without our pulling one of these near new engines down that is demonstrating this symptom and measuring and testing all facets of the components, this is the most logical theory at this stage. That said it is also acknowledged that these engines reportedly burn varying quantities of oil...some of which could be called rather excessive. But again we feel that is mainly due to the type and grade of the oil along with operating conditions more than anything else.
However, this particular intermittent smoke symptom commonly reported by owners is occurring also on naturally aspirated engines too. Therefore, logically the turbo seals must be ruled out in this particular instance. Also the problem is not limited to high mileage engines.
I recall when my own engine demonstrated the problem the first time (near new) i had the turbos inspected under warranty and all was found in to be in perfect working order/condition. No oil residue in the turbo hoses etc. The other point worth mentioning is often this smoke comes out from only one tailpipe and that can sometimes alternate. So it brings to mind the other possible causes for this type of symptom.
As you are aware the catalytic converters will arrest the emissions once at operating temperature with exception to the usual over fuelling type scenario which tends to happen in turbocharged engines during initial WOT where you will commonly see black smoke being emitted from the pipes as the engine speed/load catches up to the fuel feed, depending upon tune settings and other environmental factors of course. This is fairly standard behaviour for any turbocharged engine as i'm sure you are aware. I have yet to see one that doesn't puff a degree of black smoke out the pipes when suddenly demanded WOT. So again it comes back down to evaluating and analysing the most logical/likely causes after going through the routine processes of elimination in diagnosing the actual cause of the symptom in question.
At this stage, after discussing this particular symptom with two close industry associates expert in the field of product failure and engine diagnosis we are to the general consensus it is a residual fuel deposit issue or an inherent design issue associated to the fuel system more than anything else. So nothing to get too alarmed about we feel.
Without evidence of wear in the turbochargers or having obvious oil deposits found in the intake plenum that's the most logical answer at this stage.
So, without our pulling one of these near new engines down that is demonstrating this symptom and measuring and testing all facets of the components, this is the most logical theory at this stage. That said it is also acknowledged that these engines reportedly burn varying quantities of oil...some of which could be called rather excessive. But again we feel that is mainly due to the type and grade of the oil along with operating conditions more than anything else.