robo_porsche

I went to a slalom event today with my 996 C2 2002. I ran 6 times through the slalom course, no smoke at all. Right after the slalom I tested to accelerate to a high speed. I reved the engine to 5-6000 rpm while accelerating. I got A LOT of smoke and efter a couple of full rev pulls the CEL started blinking and the engine ran uneven. I have hade quite a lot of smoke before when attending track days but never the CEL.

I have a deep sump kit and I have replaced the AOS recently. Why do I get a lot of smoke when the other cars at the slalom event got no smoke at all? Some other cars did exactly the same things that I did...

Robert

Macster

Something's wrong with your car's engine is why the engine smokes and the engines in other cars don't smoke.

Blinking CEL is probably due to rich misfires which put the converters at risk of overheating and damage.

You'll need to read the active codes and pending codes and from these make some kind of diagnosis.

Post the codes if you want for comment.

Or just get the car flat bedded to a qualified shop for a professional tech's diagnosis.

I would advise you to avoid running the engine any at all until you get this sorted out.

RngTrtl

and your just now worried about it?

DTMiller

How many smoke threads are there where everyone tells people not to sweat it? Is the forum position now officially that smoke = flatbed to a shop? I'm confused.

perryinva

Was the intake plenum cleaned out after your AOS? When I replaced my AOS, which was working fine, I still had a fair amount of oil in the recesses of the plenum. If your AOS was bad before you replaced it, it may have had a good amount of oil left sitting in there and the hard accels just sucked it in to cylinders, causing the smoke and CELs (too rich). Macsters advice to have codes read is spot on. If it's running fine now, after some normal rpms, that may be all it is.

extanker

you may have fouled a spark plug with oil and that is causing the miss and cel. a scan tool would greatly help .i would not drive it any distance like this.let us know

robo_porsche

Yes, I have asked for advice regarding the smoke before but people have just told me that some cars smoke and some don't so it is really nothing to worry about...

I turned off the ignition right when I got the CEL. I let it settle for a few minutes, checked the oil level and then started it again, It smoked for a little while and then I reved it carefully and I got some more smoke. After 5 minutes the smoke was basically gone.

I am going to read the fault codes but I am pretty sure that they will indicate misfire (because I had too much oil in the intake obviously).

I think the symptoms are the same as if I get oil through the AOS. When I generate G forces with the car (like I did at the slalom event) oil is stocking up in the pipe. Then when I rev it hard the oil is sucked into the engine. Or are there any other pipes where the oil can get sucked into the intake? There is some kind of AOS in the oil sump too isn't there? What is that for? Could that be faulty?

Robert

robo_porsche

I have attended track days after I replaced the AOS so I think it should be cleaned out by now.

Robert

RngTrtl

saying some cars "just smoke" is like saying, "some people are just fat". In both cases it is because a reason, i.e. something is wrong.

Macster

It has always been my position -- backed by the word I get from Porsche techs on this subject -- that smoking upon cold engine start, with the smoking that is short lived and not accompanied by any other symptoms and misbehavior is nothing to worry about but I add or try to remember to add to have a qualified tech check the car out just in case.

In the OP's case the smoking was not at cold start, was persistent and accompanied by a flashing CEL and engine misbehavior.

I suspect the AOS has failed -- though while I am quick to admit I loathe the AOS on these cars I'm a bit reluctant to believe the new AOS has failed -- but that is my best guess. Thus I advise the OP to get the car flat bedded to a shop for professional diagnosis just to be safe. I do not like to run an engine when it is obviously sick enough to smoke, misfire and otherwise act up.

No engine ever got better from these symptoms by more running.

Macster

Brief smoking under track conditions is common. These engines lack sufficient scavenge pump capacity to ensure oil doesn't gather under the camshaft covers under some high cornering/braking g-force conditions.

When the g-forces go away the scavenge pump does its job but in the meantime the camshafts and camchains have whipped this collected oil into a froth.

The crankcase is full of oil vapor. Then when the engine is subjected to a vacuum and the crankcase fumes are extracted and passed through the AOS the AOS is overwhelmed and some of this vapor is routed to the intake and on into the engine where it is burned. Because the oil vapor load is so high even the hot engine and exhaust converters can't process all this oil and smoke is the result.

And in severe cases engine misfires occur.

The differences in smoking at the track cases arise from various things. One is not all AOS's are equal. Even if the AOS is new, some may not be very good, as efficient out of the box as other new ones.

If the AOS is used, it could be on its way out.

There is the oil level and oil type and how fresh the oil is. I would not run the oil level down any from the max line but neither would I run the oil level above the max line either.

As for how fresh the oil is, oil with miles on it has picked up some unburned fuel and water contamination. These lower the oil's viscosity and the oil tends to foam and froth more than it would otherwise which creates a bigger load of oil vapor for the AOS to deal with.

There are variations in engines. Some engines generate more blow by than others. More blow by generally means more vapor in the crankcase to remove.

Last but not least, there is the driver factor. Some drivers are not as smooth as others and cornering and braking generate higher than normal g forces, well, what is considered normal under track conditions. These can result in more oil collecting out of the reach of the oil scavenge pumps which then exacerbates the foaming of the oil and the generation of oil vapor in the crankcase.

Besides the AOS I know of no other path for crankcase fumes to get into the engine.

Oil can possibly get into the engine if there is an internal oil leak, which can be an valve stem seal or a piston/rings/cylinder assembly with oil control issues. While I think this rare, it is important to note how well, important it is to if the camshaft cover is ever removed to get it fitted again properly, and this includes sealing, not only so the thing doesn't leak oil out but that it has no internal oil leaks. If it does the leak eat into the engine's oil budget to be sure, but even if this is not evident, it allows extra oil to collect under the camshaft cover. While under normal circumstances the scavenge pump has the capacity to handle this under high g-force conditions it just adds to the amount of oil that is under the camshaft cover. (Under severe cases this can have the oil level in the engine's oil sump dangerously low. If the engine blows chances are the bad old bearings are blamed when of course the real culprit is the lousy camshaft cover resealing job.)

There are two de-foaming pots (swirl chambers) in the oil sump that accept scavenged oil and are designed to separate the oil and air and feed de-aerated oil to the sump. These pots are not perfect but this is why (at least part of the reason why) there is oil baffling in the sump. The baffling is there to help keep the oil intake pipe submerged in oil of course but to also only allow oil that has shed its load of air to enter the area where the oil pick up pipe is.

If the oil overtops the baffling this can allow oil with too much air into to the region where the oil intake pipe picks up oil and oil with air in it can be fed to the engine. If this happens this does not cause engine smoking but can cause engine damage.

As an aside: That risk of oil over topping the baffling is one reason why these engines should not be overfilled with oil.

perryinva

Track days would never clean out the oil in the plenum. It would only add oil to the oil already there, and then allow oil in excess of a certain level, dependent on vacuum from acceleration, to draw in the oil in the air flow path. Once enough is "skimmed off" the air will no longer carry any in to the cylinders. Basically part of the same thing Macster mentions above. The stock AOS is not really "track ready", and as mentioned, can become over loaded. They have a better, "hardened" track AOS, which if I tracked my car regularly, I would install.

robo_porsche

I talked to a shop today and they said that it could be that some of the hoses leading to the AOS are clogged! That could gererate smoke thay claimed.

I saw in the workshop manual that there are 2 AOS in the oil sump. Are they connected to the "normal" AOS (on the top of the engine) with some external pipes? What are they for? How does it all work?

Robert

Ahsai

The ones at the oil sump are to separate the air from the oil to prevent aeration of the oil, which is really bad for the engine (bearings). They serve a completely different purpose than the AOS on top of the engine, which separates the oil (vapor) from the crankcase air.

Macster

From what I've seen of the stock intake it has no place that can accumulate oil, that is allow oil to puddle. All paths in the intake run down hill so to speak.

The oil that does contribute to smoking upon startup is oil -- in vapor form -- that has passed through the AOS and unable to make that sharp turn where the hose from the AOS connects to the intake manifold impacts the opposite side of the inner intake wall from where the hose from the AOS connects to the intake manifold.

If this oil isn't pulled into the engine before the engine is shut off -- and it may not be as the intake walls can be rather cool and the oil of course becomes more viscous when cool -- after the engine is shut off the oil will drain down -- that the cold intake gets hot as the heat from the engine soaks into the intake helps in this regard -- and into the engine where it will cause smoking upon the next engine start.

What I gather from reading posts of those that have installed this motorsports AOS the motorsports AOS is a more durable unit but it is not much better at removing oil vapor from the crankcase air that goes through it than the stock AOS.

The result is an engine fitted with a motorsport AOS can still smoke at the track.

This propensity to smoke like they do is just an artifact of these engines. These engines are high RPM engines at least when pushed hard on the track and all those cams and chains whizzing around just whip the oil into a froth. Remember the cam lobes are splashed lubed and at higher RPMs this splash is really more like an oil mist. Great for cam lobe life but oil vapor is created in copious amounts. The oil scavenging only further aerates the oil and causes more oil vapor. As a result the AOS is overwhelmed and smoking occurs.