Help us help you. By posting the year, make, model and engine near the beginning of your help request, followed by the symptoms (no start, high idle, misfire etc.) Along with any prevalent Diagnostic Trouble Codes, aka DTCs, other forum members will be able to help you get to a solution more quickly and easily!
Hey guys name here is Chad. Found this forum from some youtube videos and thought i would give this a shot.
A little background, Mercedes tech for the past 10 years. Involved in automotive repair for the past 16 years.
Complaint: Girlfriends car stalled at idle. Would not restart.
2008 Pontiac Grand Prix series III 3800
Towed car home, Crank no start. only DTC P0300. Test Fuel pressure was at 59PSI with slight drop after key off. With key ON i noticed the 3-6 Coil was very hot, determined shorted coil. Replaced 3-6 Coil. Engine started but was misfiring on cylinders 3-6. Found Faulty ICM. Replaced ICM. Engine starts and fires on all cylinders. About to crack beer, engine started idleing rough and stalled. No codes. Engine would restart and seemed to run smooth, intermittently have a dropping/rough idle and smooth itself out. I also noticed that the rough idle was more severe when cold started, when the car was fully warmed up it would idle smooth untill i started bliping the throttle. Road test, Found would drive ok under light load, Found 3/4 throttle to about 4000RPM engine would Fuel cut or just cut off power. Check plugs and wires and all seem ok. Cleaned out MAF and removed Throttle body and cleaned. still runs the same, checked with the MAF unplugged same results. Replaced PCV valve. Removed Belt to check for any serpentine components locking up = OK. Check Alternator AC ripple current with scope looks clean. Checked 3X and 18X crank signal with scope. Signal present around 6-7V of amplitude. The scope i am using is a cheap pocket scope that's not very fast, i could not get a accurate reading when problem was occurring. I am missing somthing very obvious but i think one of three things, Faulty aftermarket ICM, CKP showing early stages of Death. Possible faulty PCM. Girlfriend claims since she bought the car it would intermittently start/stall at times and would start ok second time around. Engine performance was OK prior to this problem.
I agree with gav09, having some PID information would be helpful.
See if the car goes south when it tries to go into closed loop. Or is it not defendant on that?
I'm hoping your coils weren't bad for a long time since that might allow a great deal of fuel to enter your cat(s) and kill it. So, if you could check back pressure, I would.
I'm very curious to see your fuel trims and O2 sensor information.
Watch the PIDS and see if you see any of them all of a sudden act weird as the car dies. Perhaps you have an intermittent sensor. From my experience, I encountered a faulty ECT acting erratically that made a poor Honda act weird.
I suggest using water to see if there is a vacuum leak somewhere. ScannerDanner has many great videos on this. It works!
For me, I would increase the time base when looking at crank sensor to see if it is cutting out. This appears to be a three window type. But, it would be interesting to see if it is cutting completely out at times. That would mess with the computer.
If the DTC was there for a while and it is easy, I would test back pressure. I would also hook up an intake vacuum gauge to see if there is a valve train issue.
If you add fuel to intake, does the idle smooth out?
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