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Discussion Starter #1 (Edited)
Guys,

Sorry it seems there is a lot of these lately, but Im at a total loss.

Back in March/April i started getting a few misfires while driving to work flashing CEL. It started with just cylinder 1 and 2. I installed the ACdelco opti from my closed out for sale thread and had no difference. I tightened all the intake bolts (oil leaks/ misfire "seemed" to get better?) and replaced all the gaskets on the throttle body--no difference.

By the time i took the car out of service for repairs, all cylinders were misfiring 1,2 being the worst (100's of misfires) and getting less as it hit cylinder 8 (maybe 60 misfires) in a 10 minute period.

With Covid 19 going on i figured i would address the misfires and of the all oil leaks (intake and oil pan) AND of course, "while i was there" i would install a GM 845 cam.

I installed cam, injectors (purchased from forum), timing chain, replaced intake, oil pan gasket, new Autolite 104 plugs, Taylor Thundervolts, new coil, deleted EGR and a new tune from Moe. -- still have P0300. all cylinders

I replaced b1-s1 O2 sensor, looked lazy --no difference. checked for exhaust leaks at cat, none found.
IAT and CTS both read same at cold start -close to outside air.

Tri-y's make compression test a bit of a hassle, but i tested the main culprits--1,2, and 3 all at ~200psi. noid lighted all injectors -OK. inline spark tester on cyl 1 seemed good. Fuel pressure 37 psi. vac off 44 psi --installed new filter anyway.

ACdelco 3.5"--cleaned MAF multiple times--The only thing i have found is UN-plugged MAF --no misfires!! no intake ducting leaks --plugged in a warm is reading ~9-10 gms/sec (tuned at that ?) and BARO sensor seems to be pegged at 104 kpa (high i think according to google search) unplugged drops to 10kpa --still has misfires. Installed stock tune pcm for testing--no change. plugged off vacuum hoses--no change. In open loop STFT 0 both sides. LTFT -9 & +1

closed loop has more misfires than open but not by much.

EDIT--cleaned all coil mounted grounds--cleaned battery ground to block.

Sorry for the long post, and there might be something i missed, but any help or direction would be greatly appreciated.

Chuck
 

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One possible cause could be ICM. Generally the fail at operating temp causing a motor no start until the motor cools....but I have found they can also cause considerable misfire havoc

Not one for throwing parts at a problem but IMHO having a known good ICM as a spare is a good idea to rule it in/out as the problem cause

Then again it can be one of many wires involved in ignition system that has a fracture in it that is a needle in a haystack
 

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Discussion Starter #3
Thanks BALLSS, I knew I would forget a few steps I did....yes I did install what I believe to be a known good ICM. Also installed with heat sink paste from Best Buy. —same symptoms
Also swapped out Professional Products 52mm TB with factory 48mm —no change. Put 52mm back on.


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Discussion Starter #4
Does the OBD2 pcm stop reporting misifes with MAF unplugged? MAF grms/sec drop to 2.50 (from ~10 plugged in) --no misfires. MAF wiring checks out ok. Should i be looking overly lean or overly rich as my issue? Bad MAF or other sensor? wiring? O2's seem to be functioning as they should.

LTFT -15 -13 maf unplugged
STFT -7 0 maf unplugged.

IAC counts warm with maf plugged in 65 unplugged 75

Opti wiring checked ok - opti to ICM checked ok. Installed known good opti harness

engine vacuum 20 MAP reporting 40kpa

Thanks,
Chuck
 

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How old is your gas? E10 and E15 are only good for about 90 days before they start to go to ****.
 

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Discussion Starter #6
How old is your gas? E10 and E15 are only good for about 90 days before they start to go to ****.
Thanks Atlantadan, $20 worth of 93 2 weeks ago when i got it all back together....was misfiring before and after fueling.
 
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