Steven C.

Well, kind of stumped on where to go next on this one. My 02 C2 will run fine all day and then for no reason I get an intermitent rough idle. Most of the time after a few seconds it will fix itself without touching the throttle? Other times it will not until you get going to the next light and idle again...and its fine. When hot and idling in park you can induce it by bliping the throttle and most times it will fix itself again after a few seconds.

It will pop a CEL every once in a while and always misfire #6. So, replaced all the coil packs since the car has 98K on it. No difference. Cleaned the throttle body and MAF, no change. Replaced just the #6 spark plug and made it worse! Have had the Porsche computer on it and nothing really stands out. What might look bad for a while then goes away while reading the engine at idle.

Ideas appreciated.

Thanks

jrgordonsenior

Replace the MAF. Cleaning doesnt work IMHO.....

logray

Try swapping coil packs to see if stumble/misfire follows.

Check/clean/swap fuel injectors.

Test compression/leak down.

Post the exact DTCs you are getting.

logray

MAF at idle though?

Steven C.

Misfire does not travel with the coil pack. Leak down heads where I am thinking, lifter going flat? But a spark plug change making it worse makes no sense. That is why I am stuck on electrical?? I have heard that on the MAF, may be it but a lot of money to guess with.

Thanks guys!

ssukduang

I had the same problem. The dealer tried everything common to a misfire (coil, plug, etc) and nothing worked. Had it in the shop four times.

Strange enough, for me it turned out to be the flywheel. It was either slightly imbalanced, installed improperly by under the care of the po, or came slightly lose.
The dealer replaced it under CPO and it finally resolved the mystery misfire.

Barn996

Good suggestion.Cleaning my MAF only lasted for a couple of weeks. New MAF has been good for 3 years plus.

jrgordonsenior

Yep. Easy test, just unplug it and see.....

tarzancoe

Does it do it when cold? If not and only when hot, it might be a clogged purge valve that does not allow engine fumes to escape thus gas does not flow correctly. It throws no errors.

Or, gas lines?

nick49

Misfire on #6 and changing plug made it worse, coil pack no diffefence. I'd change the plug to another new one on #6

What code does this throw? Are you clearing the cel? Does it continually throw a cel and code?

I'd look at an injector. I'd think at idle it could cause a lean mixture and misfire only on one cylinder. Is condensation in fuel tank possible? Is fuel tank low? Try some Heet to displace moisture.

Steven C.

All good ideas. Next is fuel injector cleaner and heet as I have heard these before. It does only do it hot, the cold start sequence works fine. Thanks for the ideas!

Macster

The misfire error code is flagging a weak cylinder. If the misfire is not accompanied by a flashing CEL the misfire is due to a mechanical problem.

If you can with the Porsche computer you have disable the spark/injector fuel pulse to each cylinder in turn -- with the engine up to temp and idling -- and note the rpm drop.

Chances are high when you kill the #6 cylinder the rpms will drop less, indicating the #6 cylinder is not contributing as much to the engine's idle speed as the other cylinders.

The question then becomes 'why'?

What did the original #6 plug look like? I'd like to know what it looked like compared to the other plugs too. If it looks like it has been running 'hot' maybe an injector is weak and the misfires are due to a lean mixture.

If the plug is 'clean', this is a sign coolant is leaking into the combustion chamber. A head gasket failure, a crack in the head, or even a crack (D-chunk failure) is the likely cause.

A collapsed lifter should be considered, but this usually is accompanied by some noise, ticking. A flat lobe on the cam is another possibility.

If the engine has VarioCam Plus, the mechanism that provides the variable lift on at least one intake valve of one cylinder is suspect as well.

With the right Porsche computer you can activate the variable valve timing and lift feature per bank and probably eliminate or confirm there's a problem.

Sincerely,

Macster.

Steven C.

Thanks Macster Very helpful. The CEL never flashes so these are some good ideas. The plug was normal just the gaps were huge with 30,000 miles on it? No coolant loss and the car runs perfect off idle. Gas milage is down though about 1.7 mpg.

C4CRNA

Isn't #6 where the famous crack in the head usually appears?

Steven C.

Well, just to finish this thread off, THE FREAKIN CAMSHAFT IS BROKEN! So goes the most dependable Pcar I have ever owned BS. This same right side we replaced the lifters on about 2 years ago and now the end of the camshaft falls out when trying to replace a lifter. Absolutely stupid.