Sporadically running rough+misfires on track

[Jevi Javi]

i had a similar issue and was fixed changing the o2 sensors!

[CrookedCommie]

I wanted to update this thread, in the offseason I changed out everything mentioned in this thread. That included new NGK spark plugs (the old ones were pretty gapped, will now be doing these regularly), new Bosch coils, new ICV, new brake booster vacuum hoses, new pre-cat O2 sensors, new MAF, and new fuel pressure regulator. The problem is resolved, I believe the old fuel pressure regulator was bad. I was able to confirm the old one was running low, and the new one from the dealer gave me the correct pressure. To check, I got the Harbor Freight kit which had a fitting for our cars, at idle with the vacuum line plugged in it should read 65, if you unplug the vacuum line it should read 72.

Thanks for all the help everyone!

[Exclusivs]

One of the parts, yes.

Ask because; semi unrelated to OP but have noticed when AC pump turns on- (if stopped for a light, etc) ill notice pedal drop slightly.

[bigjae46]

Are you referring to this part? : 11617836745

One of the parts, yes.

[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

[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.

[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.

[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.

[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.

[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!

[Slideways]

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

[CrookedCommie]

Rudy, idle air control valve??

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

[Feffman]

Rudy, idle air control valve??

[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?

[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.