C4S misfire on 2 cylinders
04-26-2017, 08:14 PM
#1
C4S misfire on 2 cylinders
Car is a 2010 C4S (997.2). About 30K miles, PDK. I track the car heavily but well maintained. Last time at track day started out OK but during the second session I got an engine warning light, best of my memory is solid light. Reduced power but otherwise no symptoms. Came in, checked all easy stuff (like gas caps…) and reset computer with a cheapo code reader.
Next session starts out OK, problem returns. Guy next to me has a better code reader but is not familiar with it - reader shows cylinder 4 & 6 misfire.
My indie mechanic has good experience, specializes in Porsche and I have faith in him. Long story short they can consistently recreate the misfire in 4&6 but only close to the rev limiter and under load. For testing purposes this means 3rd gear on a freeway access road. It won’t do it in 1 & 2 because the engine is shifting too soon (i.e. because it needs to be under load they need to leave it in automatic and the window of rpm where the misfire occurs goes by too quickly). Can’t test in 4-6 because its on city streets. There are no 4 wheel dynos in our area and he feels uneasy putting a PDK under load on a dyno anyway.
To summarize, engine almost at rev limit under load and they get a repeatable cylinder 4&6 misfire. About half the time when it goes to idle it continues to misfire. A quick engine stop and restart and the idle misfire goes away. Does this hot or warm engine (they don’t want to wail on the engine cold). FYI there is no way to kill the engine while the car is coasting to try and keep the engine spinning while cycling just the electronics.
They have done the obvious – plugs, coils, vario cam selonoids. They cannot test the fuel pump directly, but they looked at voltage readings and everything looked OK. Compression good, no cylinder scoring.
He says the symptoms look like a stuck lifter except it would be so unusual for 2 cylinders to happen at once. Same for a broken valve spring.
He doesn’t want to diagnose by swapping parts but can’t see an alternative (he has PIWIS). His next attempt will be the fuel injectors. He is also considering the fuel pump but it seems weird that cylinder 5 wouldn’t also misfire. Things like flywheel or crank position sensor don't make sense because 5 should get a misfire too.
Thoughts, thanks?
Next session starts out OK, problem returns. Guy next to me has a better code reader but is not familiar with it - reader shows cylinder 4 & 6 misfire.
My indie mechanic has good experience, specializes in Porsche and I have faith in him. Long story short they can consistently recreate the misfire in 4&6 but only close to the rev limiter and under load. For testing purposes this means 3rd gear on a freeway access road. It won’t do it in 1 & 2 because the engine is shifting too soon (i.e. because it needs to be under load they need to leave it in automatic and the window of rpm where the misfire occurs goes by too quickly). Can’t test in 4-6 because its on city streets. There are no 4 wheel dynos in our area and he feels uneasy putting a PDK under load on a dyno anyway.
To summarize, engine almost at rev limit under load and they get a repeatable cylinder 4&6 misfire. About half the time when it goes to idle it continues to misfire. A quick engine stop and restart and the idle misfire goes away. Does this hot or warm engine (they don’t want to wail on the engine cold). FYI there is no way to kill the engine while the car is coasting to try and keep the engine spinning while cycling just the electronics.
They have done the obvious – plugs, coils, vario cam selonoids. They cannot test the fuel pump directly, but they looked at voltage readings and everything looked OK. Compression good, no cylinder scoring.
He says the symptoms look like a stuck lifter except it would be so unusual for 2 cylinders to happen at once. Same for a broken valve spring.
He doesn’t want to diagnose by swapping parts but can’t see an alternative (he has PIWIS). His next attempt will be the fuel injectors. He is also considering the fuel pump but it seems weird that cylinder 5 wouldn’t also misfire. Things like flywheel or crank position sensor don't make sense because 5 should get a misfire too.
Thoughts, thanks?
05-23-2017, 09:49 PM
#2
It took about a month but a solution appears to have been found. I will know for certain after the next HPDE event this weekend.
What we think happened is that the coil on #4 was failing. When it failed at high rpm on track, it took out the cat for that bank. We know for sure the coil was failing and the hole in the cat was impressive.
We cannot figure out why, but even after replacing the coil the damaged cat would cause a misfire on 4&6 as originally described. They replaced the fuel pump and the injectors and the misfire remained. Replaced the cat and the misfire went way and that includes returning to the original injectors and fuel pump. For comfort we replaced all the coils on 4-6.
No good explanation about why #5 never misfired or why there would be the intermittent misfire at idle.
What we think happened is that the coil on #4 was failing. When it failed at high rpm on track, it took out the cat for that bank. We know for sure the coil was failing and the hole in the cat was impressive.
We cannot figure out why, but even after replacing the coil the damaged cat would cause a misfire on 4&6 as originally described. They replaced the fuel pump and the injectors and the misfire remained. Replaced the cat and the misfire went way and that includes returning to the original injectors and fuel pump. For comfort we replaced all the coils on 4-6.
No good explanation about why #5 never misfired or why there would be the intermittent misfire at idle.
