lincolnturns

(UPDATE - 12/18 - fresh plugs gapped down to .026 and FQS on setting 2 - pulling 3% fuel has seemingly made all the difference in the world )

GPA951s

You have allot going on with aftermarket parts, But as a General observation I would look toward the injectors, I had a few injectors go bad. Sounds like when the car is cold the Fuel is keeping the injectors cool and working well. Once the car is shutoff the residual heat "Soaks" the injectors and they start messing up.

lincolnturns

Symptomatically your bad injectors showed similar signs? I'll hit the old Googler here in a minute but what is the procedure to check the injectors? They have relatively low miles on them since install.

lincolnturns

Or reading about DME temp sensor? Sounds like that might affect cold start more than anything but maybe reverse symptoms are possible?

Jeff N.

Hey...checked my notes. Looks like there is only one engine temp sensor so my bad on that one. I think getting an AF reading while the problem is occurring is a key next step. Options will become clearer once you know if it's lean, rich or in range. Fouled plugs - as mentioned above - does suggest a rich condition...confirmation helpful.

bw993

If you have an M-Tune and you are running rich only when hot, check the M-Tune temperature sensor that is next to the MAF sensor in the intake. Does your car only run rough after a re-start, when the engine is hot? This is the part that is weird, since the M-Tune temperature sensor is used in the calculation for the air density for the MAF. The only other temperature sensor used by the DME, is in the block and it is used to determine whether the engine is cold to adjust the starting fuel mixture for cold or hot starts (i.e. "choke").

lincolnturns

Yes bw993 the car only seems to run rough upon hot engine restart. First time I fire up the car and even when it get up to operating temp during the first drive it runs great. Only after shutoff and apparent heat soak does it run poorly.

bw993

The Mtune software does increase the fuel mixture due to hot ambient temperatures. Josh said this was to reduce detonation when the temperatures were >85F. He added enough fuel to drop AFR by 0.5. Check your AFRs.

lincolnturns

Will do, but happens regardless of outside temp - being super cold out or hot and sticky.

fejjj

lincolnturns

Much appreciated fejjj. New PC arrives Tues and I'll pull some logs and then look into the MAP sensor. I tested it independently through the LR procedure previously and readings were correct but I'm realizing that was on a cold motor.

fejjj

lincolnturns

Makes sense.

lincolnturns

Bringing this back up top as I've spent some more time digging. Replaced MAP sensor with new GM version as suggested and replaced DME Fuel Pump Relay. Car seemed to run great initially but after two subsequent drives the mystery problem persists. It runs stellar upon first cold start and I can drive 5-20-60 minutes with no issues. Pulls like a freight train, idle is okay etc. Max boost on my setup is 14-15 psi and it holds great. Trying very hard to look down at the AEM wideband gauge but tough when in the throttle. Upon letting off and decel it pegs itself at 19+ ( - - ). Otherwise mid throttle and cruising it seems to seriously bounce all over the place. 13-17 constantly.

Today I drove for 45 minutes. Let the car sit for 30 min and then headed home. Idle was lumpy upon restart but it fires up first try. Then immediately when I try to get into the throttle - it coughs and backfires. (Stumbles/hiccups?) Even 1psi of boost will cause stumble. I can let off and roll into it 3-4 times and get past this and it will build boost and pull hard but then after letting off and coasting for a while it repeats. Seemed to slowly get better during the 40 minute drive home but still had a few instances during the drive (doesn't totally go away). Highway cruising is eventually totally fine, pulls in 4th and 5th under load.

Would love to keep this conversation going if anyone else has ideas. Still cannot data log as the ProTuner software and data logger will not connect to the car no matter what I try.

Tom M'Guinn

I'd put someone in the passenger seat and have them video the wideband as you drive. Sounds like you are going rich if the plugs were "moist" --unless it was coolant -- so now is the time to get your money's worth out of the wideband. Almost sounds electronic, but I can't think off the top of my head of a part or circuit that might fail when hot but not matter unless it fails when you are starting up.... Did you check the FPR for gas in the line? Seems possible fuel is keeping something cool enough while driving, but it then fails when sitting hot.