2000 Grand Prix SE 3.1L V6 - Inconsistent DTC P0304 FIXED

[premierautohandling]

Six months ago customer needed head gasket replaced due to coolant spewing out just under the throttle body where the lower Intake mates with valve covers and heads. No problems during R&I. Car performed as it should after repair. The car has only 53K miles.

Last week customer called and said the SES light began to flash while driving and the car struggled and almost stalled. I pulled DTC P0304 only. HISTORY MISFIRE counter was through the roof. With my Verdict scanner connected, live data showed only two periods of misfiring that amounted to 4-6 misfires per cycle with 10 minutes or so between periods. I monitored live data for a half hour of idle and stop-and-go driving.

FYI - Plugs and plug wires were not replaced when gaskets were replaced due to good condition.

After the drive test and live data monitoring, I pulled Cyl. #4 plug and found a .085 gap. Plug was dry and very little carbon deposit. Emission sticker on hood listed gap of .060. I regapped plug and reinstalled. Ran another live data scan and misfires only got worse - strill only on Cyl #4. I double checked wire condition and it checked out fine. So I simply replaced plug with a new plug and misfires decreased, but returned to previous behavior like old plug prior to regapping.

During a period when no misfires were being displayed I performed an Injector Balance test. Fuel pressure read 50 psi prior to pulsing each injector; during pulse, pressure dropped smoothly to about 30 psi and quickly returned to 50 psi after pulse. Each injector, even #4, had same results. I would still get misfires later on #4 only.

From there I have swapped wires, coils, plugs, checked and double-checked vaccuum hoses, performed a compression test and still get same results. The only variable that made an impact during a misfire period was putting pressure up against the injector with a screw driver attempting to move or shift the injector. After closer inspection, I found that the retaining clip of the electrical wire to injector was not fully clipped - so were all other wires to injectors. After clipping all wire harnesses, live data still showed misfires on Cyl #4 as before.

Should I assume injector #4 is faulty and replace it or is there something else I can try?

[steven kiser]

that or a bad pcm. cars of the vintage i'll reflash the p.c.m before i jump into anything else.

[Tim Martin]

Having worked on tons of these 3100s, I ain't saying I am good but I did learn a few things. Beings that cylinder #4 is the middle cylinder on the front row, there is no vacuum lead close by, I'd be leaning towards a shifted lower intake manifold gasket or an injector issue. Due to the cost of injectors, I'd try switching two injectors, say #4 & 6 an see if the miss goes to #6, If it does, there is yer problem. I will say this, poor electrical connections can cause issues on most any sensor or controlled output in todays engines.
Back to the intake manifold gasket issue. I don't know what brand of gasket you used but I have found that the only ones that work for us are the more expensive ones from Felpro. They have 2 lines of gaskets for these engines and it usually pays to spend the extra 20 dollars or whatever it is to get the better gaskets. I am not sure of the part number without looking it up. I also have used the oem gasket with some failures and will not use them.
A white plug could indicate a lean running cylinder. I have also seen a cracked intake manifold already on this engine, strange as it seems. How does the #4 plug compare with the other plugs?

[steven kiser]

totally agree and i don't know why i didn't pick up on the plug being so clean........

[Tim Martin]

that or a bad pcm. cars of the vintage i'll reflash the p.c.m before i jump into anything else.

Yea you could try reflashing the pcm at this point. The only thing I'd say is that I don't think I have seen an intermittant miss created by the pcm. But I suppose anything is possible.

[steven kiser]

more i think about it the skip caused by a p.c.m. issue is consistent but will change from cylinder to cylinder. it's not intermittent.

[Tim Martin]

ahhhhhh steve. Pardon me but I thought the original post was indicating the miss stayed on #4. I will have to review it and get it straight.

[steven kiser]

to clarify a bit. a pcm issue will not (at least i've never seen on these, an intermittent misfire) it will however cause a misfire that will be constant and on occasion switch cylinder. it's about the best description i can give. you gotta give me break. i've been outside in this heat replacing king pins on a truck and been overheating so long i'll argee with my wife without a second thought....................... it's been a bit toasty. i've actually had to sit down and take a break, scared the guys and my wife. they've never seen me just sit and take a break..............

[premierautohandling]

Thanks guys for the suggestions and input. I'll try both a reflash of the PCM and/or swap injectors and see if doing so creates a different outcome.

[Steve 3871]

THIS IS A VERY INTERESTING ISSUE, THE PLUG IS CLEAN? IS IT WASHED FROM COOLANT INJESTION? I BELIEVE THIS VEHICLE HAS 6 INJECTOR DRIVERS, SO
ONE INJECTOR THAT IS OUT OF SPEC (OHMS) WILL NOT SHUT DOWN THE PCM CAUSING A NO RUN CONDITION. I WOULD CHECK SPECS AND OHM INJECTORS BEFORE SWAPPING THEM. ALSO I WOULD INVESTIGATE THE CLEAN PLUG ISSUE. GOOD LUCK, Steve

[premierautohandling]

Replaced injector...

[premierautohandling]

FIXED

[steven kiser]

congrats