Random Misfires
05-21-2011, 08:39 PM
#17
Rennlist Member
I bought a set of Fabspeed 200 cel cats second hand (never previously installed) and got a check engine light. Fabspeed gave me a "discount" on a replacement set. Said the cat internals were "coming apart" and could be "risky" to drive the car. No check engine light with the new cats and power seems good. Consider talking to Fabspeed about the problem...
05-22-2011, 05:52 PM
#19
Race Director
I tested out the MAF John sent me (Thanks John!) today and it didn't fix the issues. Basically, when I hit 6k+ rpm the check engine light comes on. But, the car isn't running right at normal RPM's either. The codes are random misfires in cylinder 4 and cylinder 5 each time I have them scan and reset. I changed the airbox back to stock prior to having the codes reset and testing the MAF. I think my next move is going back to the stock exhaust and headers - back to ground zero with no engine mods. Any other ideas are greatly appreciated.
I'd look them over and if any look bad replace at least those and an argument could be made to replace all of them if 2 say are bad.
OTOH, 4 and 5 are on the same bank and this bank gets the worst of the road splash so maybe you could get by with just replacing 4 and 5 though with 2 out of 3 bad on one side I'd replace 6 too.
If you want to verify the coils switch 4 or 5 (or both) with coils on the other bank and see if the misfires follow the coils.
I'd consider doing the above before I'd do to the trouble of reverting to the stock exhaust, etc.
Sincerely,
Macster.
05-22-2011, 06:46 PM
#20
Rennlist Member
Thread Starter
What others have suggested: coils.
I'd look them over and if any look bad replace at least those and an argument could be made to replace all of them if 2 say are bad.
OTOH, 4 and 5 are on the same bank and this bank gets the worst of the road splash so maybe you could get by with just replacing 4 and 5 though with 2 out of 3 bad on one side I'd replace 6 too.
If you want to verify the coils switch 4 or 5 (or both) with coils on the other bank and see if the misfires follow the coils.
I'd consider doing the above before I'd do to the trouble of reverting to the stock exhaust, etc.
Sincerely,
Macster.
I'd look them over and if any look bad replace at least those and an argument could be made to replace all of them if 2 say are bad.
OTOH, 4 and 5 are on the same bank and this bank gets the worst of the road splash so maybe you could get by with just replacing 4 and 5 though with 2 out of 3 bad on one side I'd replace 6 too.
If you want to verify the coils switch 4 or 5 (or both) with coils on the other bank and see if the misfires follow the coils.
I'd consider doing the above before I'd do to the trouble of reverting to the stock exhaust, etc.
Sincerely,
Macster.
05-31-2011, 09:57 PM
#22
Rennlist Member
Thread Starter
And a new update...
There was some cracking on the surface of one coil - swapped it out and saw no improvement. But, there is a new twist. I did a fuel pressure reading and it is high - 4.4. The book says it should be 3.3. Could these be related?
Banging my head against the wall looking for answers.
There was some cracking on the surface of one coil - swapped it out and saw no improvement. But, there is a new twist. I did a fuel pressure reading and it is high - 4.4. The book says it should be 3.3. Could these be related?
Banging my head against the wall looking for answers.
05-31-2011, 11:28 PM
#23
Race Director
And a new update...
There was some cracking on the surface of one coil - swapped it out and saw no improvement. But, there is a new twist. I did a fuel pressure reading and it is high - 4.4. The book says it should be 3.3. Could these be related?
Banging my head against the wall looking for answers.
There was some cracking on the surface of one coil - swapped it out and saw no improvement. But, there is a new twist. I did a fuel pressure reading and it is high - 4.4. The book says it should be 3.3. Could these be related?
Banging my head against the wall looking for answers.
(This is for the 996 Turbo and the 996 GT2 and generally where the two differ the differences are noted.)
At idle the fuel pressure value should be 3.3 bar +/- 0.2 bar.
Be sure you replace the brass closure cap on the test connection of the fuel pressure line after using it cause the sealing ring can't be replaced. The cap must be replaced after removal.
A too high fuel pressure can cause the engine controller grief because the length of the fuel injector pulses and thus the amount of fuel injected into the engine is based on a fixed and known fuel pressure.
With too much fuel pressure too much fuel will be injected.
The engine controller will detect this and cut back the injector pulse widths. The short term fuel trims will go negative as the engine controller leans out the fuel mixture. If the trims go too far that's an error the fuel trims are at their lean limit.
If the engine controller is able to adapt before this limit is reached, the long term fuel trims will be updated with new lean trims and the short trims will revert to their default setting.
Now if the fuel overpressure is temporary and the fuel pressure drops back to normal the fuel mixture will be too lean and this can result in misfires before the engine controller can react to the change in fuel pressure.
So, I think you may be on the right track. Do you have confidence you accurately measured the fuel pressure and found it high and are you sure of your source of info as to what the fuel pressure should be?
(As I noted above, the info I have is for the Turbo and GT2. The GT3 may have a different fuel pressure call out.)
Sincerely,
Macster.
06-01-2011, 09:49 AM
#24
Vacuum leak. Probably when the intake was installed. Just a thought.
Same thing happened on my M3 when I swapped the intake manifold. The slightest leak caused random misfires. Replaced the dry rotted elbow and resolved the issue. The code was very misleading. Only happened under load, as well.
Same thing happened on my M3 when I swapped the intake manifold. The slightest leak caused random misfires. Replaced the dry rotted elbow and resolved the issue. The code was very misleading. Only happened under load, as well.
06-03-2011, 12:23 PM
#25
Rennlist Member
Thread Starter
I think we have resolution - looks like it was all down to a bad Fabspeed cat that I probably blew at the track. Should have put the stock exhaust back on before checking the other stuff. Bad news is I had a lot of wasted down time. Good news is I have new coils, new fuel pressure regulator, and I'm ordering cat bypass pipes. Thanks everyone for your help, I'll let you know if the problem comes back.
