Sporadically running rough+misfires on track

[CrookedCommie]

Hi everyone, I have the following weird issue at VIR over the weekend. Hoping to get some pointers/tips on what to look for.

While on track into a session, the engine would start to run low on power and rough for a couple turns, then everything went back to normal after about 30 seconds- back to full power. Running codes would give me cylinder misfire 1,2,3,4,6 codes as well as the multiple cylinder misfire code. Only other error code was for EGT sensor - which was replaced with a brand new one on Saturday. I replaced the 6 coils with new Duralast ones, but did not change anything. I found the vacuum line at the back of the fuel rail was unplugged, so got that fixed, ran two full sessions with no issues, but then ran into the same problem on the next session. I checked grounds, did not seem like any were disconnected. The fuel system is pretty much new except for the fuel pressure regulator. The car ran with no engine issues the previous weekend.

So far I have the following to check:
Check vacuum lines and airbox
Replace fuel pressure regulator, double check fuel pressure
Put in 6 new Bosch coils and spark plugs (4-5 seasons old)
Check wiring and grounds in engine bay, fuse panels, etc.
Maybe not a bad idea to replace all the O2 sensors? They are about 4-5 seasons old.

Any other suggestions or tips are really appreciated. This one may be hard to diagnose, as I'm not sure how to recreate it. It would happen 20 min into a session, 10 min, sometimes as soon I went out. After each time I pitted as I did not want to keep running the car and turning into a rolling chicane.

-Rudy

[foolio]

I had a really strange misfire on track issue recently. Only happened on left turns. Turns out my intake boot was not fully seated on the airbox and left turns ever so slightly pulled it away and opened a gap sucking in massive amounts of unmetered air causing the ecu to violently cut fuel. It was like the engine was coughing and misfiring. Tightening the clamp fixed all.

[CrookedCommie]

I had a really strange misfire on track issue recently. Only happened on left turns. Turns out my intake boot was not fully seated on the airbox and left turns ever so slightly pulled it away and opened a gap sucking in massive amounts of unmetered air causing the ecu to violently cut fuel. It was like the engine was coughing and misfiring. Tightening the clamp fixed all.

This sounds similar to what I experienced. I will go through and check the airbox - I was in there two weekends ago replacing a throttle actuator and it may have taken a weekend to open something up for unmetered air. Thanks C!

[thegenius46m]

Is the car worse in vacuum than at wot? These cars usually are in my experience. Does unplugging the maf help the idle and stop the misfiring? Car would run rich to comp for unmetered air. How old are your injectors?

[CrookedCommie]

Is the car worse in vacuum than at wot? These cars usually are in my experience. Does unplugging the maf help the idle and stop the misfiring? Car would run rich to comp for unmetered air. How old are your injectors?

Injectors got cleaned 2-3 seasons ago, it doesn't have the problem at idle so far. Just on track. Not sure about vacuum at idle vs wot, need to check how to measure/test that.

[thegenius46m]

Injectors got cleaned 2-3 seasons ago, it doesn't have the problem at idle so far. Just on track. Not sure about vacuum at idle vs wot, need to check how to measure/test that.

Vacuum is nearly 0 at WOT and max at idle. The more throttle you give the more vacuum gets depleted. So if you have a vacuum leak the car will run better at wot and struggle at idle. These cars are even worse below 4k with a vacuum leak because the icv is pumping air into the itbs to keep the car running nicely. If you had a vacuum leak the car would be worse at anything but wot. If you could measure afr with a wideband it would tell us a lot as to what is happening. In my experience though, multiple cylinder misfire is usually a fuel or spark issue. Unlikely to be spark plugs or coils because the chance of them all failing at once is just unrealistic unless the engine is totaled. Test your precats with ISTA and if its not that look into TPS or maf maybe?

[Feffman]

Rudy, idle air control valve??

[CrookedCommie]

Rudy, idle air control valve??

Could be, never replaced that - only cleaned. I'll check that out as well.

[Slideways]

You said both Bosch coil packs and spark plugs. If they are Bosch plugs, recommend going back to the stock NGK plugs.

[CrookedCommie]

You said both Bosch coil packs and spark plugs. If they are Bosch plugs, recommend going back to the stock NGK plugs.

Meant the coils - I believe the plugs are NGK so going to replace with the same ones. Thanks!

[thegenius46m]

Could be, never replaced that - only cleaned. I'll check that out as well.

ICV definitely goes bad over time even if its clean. On track I'm not sure this would matter heavily because the car mostly uses it below 4k rpm to pump air into the itbs.

[Slideways]

Do the misfires happen with a full tank of fuel? If the tank is low and the jet pump tube has fallen off the driver's side level sender unit, fuel won't be getting back to the pump while cornering and could cause a fuel cut and possibly misfires.

[CrookedCommie]

Do the misfires happen with a full tank of fuel? If the tank is low and the jet pump tube has fallen off the driver's side level sender unit, fuel won't be getting back to the pump while cornering and could cause a fuel cut and possibly misfires.

Yeah I suspected that - the car has a new main pump and a twice replaced now BW secondary pump. I'm like 99% sure it's not a fuel issue, unless the fuel pressure regulator is soft failing or there's an issue with an injector, although I'm not sure how to diagnose the injectors. Tested with full tank after the initial fail, and there was no change unfortunately.

[bigjae46]

You can't really clean the ICV. Sludge gets into the bottom of the valve which you can't really get to.

If you're looking for air leaks...one thing to check is the brake booster vacuum loop. You can get leaks in the diaphragm inside the booster, a solenoid that is not functioning properly or a leak from the 3 way valve. I pulled that whole line out to replace the booster and the 3 way valve literally fell apart.

[Exclusivs]

You can't really clean the ICV. Sludge gets into the bottom of the valve which you can't really get to.

If you're looking for air leaks...one thing to check is the brake booster vacuum loop. You can get leaks in the diaphragm inside the booster, a solenoid that is not functioning properly or a leak from the 3 way valve. I pulled that whole line out to replace the booster and the 3 way valve literally fell apart.

Are you referring to this part? : 11617836745