I am a newbie on this site, as well as to my '05 997 C2.

I live in Canada, and the car is from the US. I went down to check the car, and everything seemed to be fine. From that time until i picked it up, it sat, unused for about 6 weeks. And apparently another 8-10 weeks prior to me going for a test drive. When i finally picked it up, as i fired up the engine, i got a CEL. For the sake of keeping this brief, i will not get into the conversations i am having with the person i bought it from.

Scanned the DME and got misfires on cylinders 1, 3, and 5. At this point i changed all spark plugs, air filters, oil, and ignition coils.

Next morning, CEL again on a cold start. Scan now showed cylinders 1, 3, 4, 5,and 6. Erased these codes, and thought i would just drive it off, since the car has been pretty much untouched for at least a few months.

4-5 days later, cold start CEL, this time misfire in clylinder 3 only. Erased the codes, and continued driving.

Drove for another 5-6 days, and brought it to my local Porsche dealership. Although there is no CEL, the codes are still coming up. This time Cylinders 1, 4, and 6.

Dealer checked fuel pressure, and all is ok. They charged me $500 and said they have to do a leak down/Compression test and check with bore scope, and they would charge an aditional $700-800. I told them i would pick up the car and think about it.

The car does run a little rough at idle, but other than this, it runs great. Even with the CEL on, there are no physical signs of misfiring.

Does anyone know what this could be?

I have some exerience with diagnosing misfires, and i dont want to "trial and error" this one.

Thanks for your help!

 

i just went with misfires myself. here is full list of issues, probability from high to low:
1) dirty/bad MAF sensor - VERY probable for misfires on all cylinders at once. test is simple - disconnect power, disconnect cord from MAF sensor, start up car and drive around not going higher than 4K RPM. if misfires stopped and car sounds OK - it was MAF.
buy MAF cleaner first prior to buying new sensor and pour whole can into MAF sensor, from all angles, inside, out, etc, but do not touch any parts. with some luck it may help, or may not. if you have durametric - at ignition switched on MAF data reading should show below 1.1v or so as I understood.

2) bad coil/coils. that was my case - bad coil on 2nd cylinder. if you do not have spare coils and have consistent misfire on one bank and not other - swap coils and see if misfire moves to another bank.

3) valves - stuck valve lifters. bad luck issue, big $.

4) bad DME - people say sometimes it happens, DME just go bad, also quite big $ to replace.

search around on 996 forum for this stuff, it has more info, also search on renntech site.

 

I have the same issue on my 05 c2. New plugs ,coils made it run better. The mis fire only happens on low rpm, otherwise it runs great. The car sat for 3 wks while i put a wing on and when i started it, the mis fires started. My local indie thinks it might be the ethanol in the fuel clogging up the injectors. The reason he says that is, he spoke to a porsche mechanic in Fla. and he told him that there has been a lot of problems with ethanol in porsches. Now the stupid Gov't wants to boost it up to 15%.I,m thinking about running fuel injector cleaner through under pressure to clean it out. BTW how do you check the DME?

 

Couple of things stand out. The car has been little used for a long time. Has it been used enough to go through the gas that was in it when you first looked at the car?

Have you added any fresh gas to the tank?

Under what conditions was the car allowed to sit unused? I'm always concerned about rodent infestation when a car sits unused for long periods of time.

You've replaced the components that come under suspicion when misfires appear. When you changed the oil did the old oil look ok? Did you check the oil from the oil filter housing and did you examine the oil filter element for anything scary?

Anyhow, that gas may be really really stale. And if it has ethanol in it over time the ethanol and gas can separate, unless you tell me you've run that gas out of the tank and have filled the tank since.

If the check engine light is not blinking the misfires are not converter damaging which means the misfires are not the type to dump raw/unburned fuel into the exhaust.

This suggests that the injectors are ok -- that one or more are not leaking -- and brings me back to the gasoline.

Unless you have run enough gas through the engine to burn all that old/stale gas I'd do that or even add gas or even worse case arrange to have as much existing gas removed from the tank and then fill up the tank with fresh gas and add in a good gas additive to help take care of any water (unlikely but you never know) and simply clean the fuel system and injectors and so on.

Unlikely the MAF is dirty unless the air filter is non-stock or stock but not installed properly.

You can clean the MAF if you want. It doesn't cost much and take much trouble but you want to handle that MAF with care.

It is rare but a bad MAF can cause misfires.

What you can do is disconnect the MAF at the wiring harness and clear the error codes (to reset the long term fuel trims among other things to their default values) and then drive the car.

You may still get a CEL (cause the MAF is disconnected) but if the misfires are not present... MAF. However, connect the MAF, clear the error codes again and verify the misfires return.

In all cases be sure you drive the car more than just around the block. Roughly you should drive the car 30 miles or more in varying types of driving to subject the engine to as many various operating scenarios as possible to give whatever is causing the misfires a chance to appear.

Be very attentive to the engine while you are driving the car and if the symptoms get worse, if the engine starts making any odd noises at all, shut off the engine first and ask questions later.

Sincerely,

Macster.

 

I'd not do any pressure cleaning of the injectors just yet.

Buy a bottle or two of Techron and use it according to instructions.

After you've run the last tank of gas with Techron in it down to 1/4 tank level arrange to change the oil/filter.

Checking the DME is done by various means depending upon the error codes associated with the check engine light and the symptoms that are being manifiested.

Best to eliminate other things first before go after the DME. They are expensive to replace.

If you test one if you are not careful you can damage it. And if the DME is bad it may have been damaged by something else so you can install a new DME and this somthing else can damage the new DME.

Best to assume the DME is ok and by a process of intelligent diagnostics eliminate the more likely and least costly to address possible causes first.

Sincerely,

Macster.

 

Macster,I have run enough gas additive and fuel injector cleaner through the car to drown a horse. Techron, seafoam and BG44 was run @ double strength.and still have misfires. I have had the blinking cell and solid cel light but stays solid for now. I have cleaned and replaced the maf sensor.I have run plenty of gas through the car.Air filter is new.Can you install an airfilter wrong and have mis fires Just went by to see my mechanic and his scanner shows mis fires on # 1,5,and 6 cylinders. He wants to swap out the injectors to other cylinders to see if that is the problem, but like i said the injectors are new.Thanks for your input.

 

also, it was somewhere written before but it is a pita to find this info - for checking MAF sensor with Durametric - with ignition on but engine not started your valtage from MAF sensor should be below 1.1v. Mine shows 1.01v.

When you start up your engine - at idle MAF data in kg/l or something - 2 positions higher than MAF voltage in durametric list of metrics - so at 1K or bit below at idle it shows value of 15-17. when rpms go to 3K it reads about of 75-77.

 

also look at 'Engine Roughness' parameters in durametric. As I know now even when my engine ran with no cels cylinder 2 was a bit higher there and alas - now with new coil it blends in perfectly with other cylinders.

 

So how long has the gas been sitting in it.

I'd have the engine scoped.

 

gio-73,Are you going back to the dealer? Closet dealer to me is 90 miles away. It sounds like we
may have similar problems.Let me know what you dealer says. Thanks

 

This sounds to be very similar to a recent issue I had with my 2007 997 C4 (90K+ miles). It started with an intermittent flashing CEL on cold start-up that turned into a solid CEL. Took it to the dealer who replaced the heavily worn coils. One coil and all of the plugs had been replaced several months earlier. Picked-up the car, and the problem returned soon after. Took it back in, the dealer removed the valve cover on the passenger side bank and replaced the lifters and some valve springs that were loose. Still had misfires on passenger side bank. Swapped the cam sensor from the driver's side bank, no improvement. Replaced the cam shaft. Passenger side bank okay but now misfiring on driver's side. Replaced the other cam sensor. It's been fine for 1,500 miles now. Diagnosis and fix took a while. Fortunately for me, another customer brought a car in with a similar problem during the time my car was there, increasing their motivation to diagnose the problem. A spendy problem on a previously very reliable car.

 

Ok. You've eliminated the gas as being the cause.

My WAG is a mis-installed (even absent) engine air filter would not cause misfires, though I have to point out if the intake system is compromised and is somehow picking up heated air from inside the engine compartment this can cause some hard to trouble shoot symptoms. This might include misfires.

If the fuel injectors are to blame either they are blocked and restricting fuel or delivering an unsuitable spray of fuel, or are dirty or worn or damaged and leaking fuel.

All the DME has to go on is the readings from the O2 sensors and if one or more injectors are say leaking on one cylinder bank the sensors for that bank will indicate this. The DME will detect a lack of oxygen in the exhaust gas and the DME will lean the cylinders of that bank out (shorten the fuel injector pulse times). If just one injector is bad that cylinder continues to receive a reasonable dose of fuel while the other two cylinders are leaned out and misifires result.

The long term fuel trims would be of some interest but these get reset when the check engine light is extinquished (the error codes are erased) and the engine needs to be run some for the long term fuel trims to be changed.

I do not like running a sick engine unless absolutely necessary. If one injector is leaking this can dump fuel in the cylinder which washes away oil which can over time lead to excessive bore/cylinder wear.

So, you can swap injectors. Or you can just replace them, though I do not like to throw parts at a symptom unless compelled to.

There is another possible course of action: You can remove the injectors and some shops a fuel injector cleaner/tester cabinet in which the injectors are placed and connected to fluid lines and there is a pump that pumps a fuel injector cleaner/test fluid through the injectors. Then one can view the spray pattern and observe the quantity of fluid each injector passes over the same span of time as all the other injectors.

When the 6 injectors are subjected to this test the underperforming injectors are obvious.

Another possible cause is the VarioCam Plus system on one or both banks is not working properly. With proper test equipment (PST2 or PIWIS, probably PIWIS in the case of the 997) the tech can activate/deactivate these features on a bank basis and observe the DME as it attempts to adapt. How the DME adapts, the resulting readings from various sensors (O2 sensors for example) the tech can know the VarioCam system (variable intake valve timing and variable (full or low) lift) is working properly, or not.

Sincerely,

Macster.

 

Macster,
I appreciate your attention to my post.

Both when i first inspected the car and after i picked it up, the tank was at less than the 14/ mark. The misfires continued right after i filled the tank with fresh fuel.

Prior to me picking up the car, the car sat in a showroom for most of the time, and in an outside parking lot for about a week.

My mechanic did not mention anything out of the ordinary with the oil and/or filter.

So far, about a week has past, and even after numerous cold starts, the CEL has not come back on. I'm on my 3rd tank of gas, and i ran fuel treatment in the last two full tanks. I think this, coupled with a few longer trips on the highway may have solved the problem. I'm keeping my fingers crossed.

In the meantime, i'm falling more in love with my 911 with every ride...

I will definitely keep everyone posted with the progress.

s2Rick,
I dont think i will be visiting the stealership for a really long time. The intention of my visit was so they can diagnose the problem and give me copies of all the service records. Instead, they charged me $500, didnt even give me possibilites of what it could be, and not to mention, im still waiting for the records.

 

I am aware of a similar situation where cold start misfire CELs were a challenge to resolve on a 2006 C2S.

The repair effort was focused on apparent cam timing variances.

A voltage variance (I don't know the details) was finally detected between cam banks and it was determined that an incorrect power lead had been tapped during a PSE retrofit.

 

Bad fuel gets the blame often when it is not the blame, but sometimes the blame is justified and I think based on what you've posted it is justified.

The fuel may not only be stale but depending it may be the wrong blend for the weather conditions in your area or for the altitude in your area.

For instance in many areas of the USA fuel is blended differently depending upon the season.

So, fill up the gas tank towards the end of summer with a fuel blended for summer driving then let the car sit unused for some months not only is the fuel stale but it is a 'summer' blend that finds itself in a fall or even a winter situation.

'Bad gas'. It is not really bad (well it is likely stale) but it is the wrong blend and stale on top of this... engine misfiring can result.

Also, in some areas of the country -- high altitude areas --- all you can buy is 90 octane gas. As one drives his car higher up the engine's octane requirement drops so this 90 octane gas is ok, as long as one remains the higher elevation area.

But leave the mountains for the plains, or lower elevations, that 90 octane gas is no longer sufficient.

Granted in many areas the country one can't drive from say 10K feet above sea level to sea level on one tank of gas but if the car is shipped...

Sincerely,

Macster.