Thread: Random multi misfire on 01 grand prix gt

In short, my roommate car is misfiring for an unknown reason still, parts have been replaced.... still misfire but very lightly compared to before, engine light is on and blinks when in drive "no change before or after part swap".
Details done as follows.
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01 grand prix gt 3.8
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Roommate asks to look at car.
I hooked my diag computer up. pulls random multi misfire code. - (I'll need to add the code later as I do not have access to vehicle now, house keys n such on his ring are needed)
------------------------------------barred him from driving at this point------------------- He can use my car, for the time being..
Start with basic plugs and wires. - Car was bought used, these looked to be stock from manufacturer.
Start car after install, same error, same stutter in misfire..
So now I take the ignition control module in for testing, fails horribly in a few locations just to be sure...
So now i drop in a new icm plus 3 coils
Misfire is MUCH lighter now but still here... - engine light on still and blinks while driving still
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Can anyone point me in the right direction?
I assume ill need to check fuel line pressure? but seems to be getting gas fine... not hard starting either..
I cant seem to find any vacuum leaks via starting fluid test method.
He had installed a brand new battery prior to problem rising..

I dont want to go replacing everything as I go, but will if needed....
I also tried to look up info and call several places around, most say just drop it off. My roommate doesn't wanna do that because i can fix it myself to cut costs...
Is there some sort of sensor on this engine that I may have missed?

Thank you for your time!
Have a great day!

172,000 miles rounding up. as for the cat challenge, I cant do that till I get off work tonight.
Thanks for the tips

Update!
Cat does not turn red hot when driven hard. rescanned for codes and they changed a bit from the original codes I had.
CEL Codes are:
1: P0172 - system to rich Bank 1
2: P0304 - Cylinder 4 misfire
End..

I had Random Multiple Misfire before I started.
I drove it myself tonight, it misses for a bit. then doesn't for a bit....
kinda shaky when acceleration + miss happens, othewise it acts normal when it doesn't miss..
This is maybe a minute between miss and not miss conditions.
If I accelerate fast, and bring it to a steady RPM at a constant , it runs smooth and no CEL comes on and error codes are gone as well.

Thanks for your time!

again?. i just had plugs, wires, coils, icm installed. surely it couldn't be fouled already

weird... the plugs i took out where bosch double platinum plugs before the problem. i have installed double iridium now. ill go pick one up for that cylinder....

well, no change. cylinder 4 misfire still.
took plug out, it seemed fine..
i changed it with a new one anyways, and took off the metal boot shield just to rule that out..

tried 3 other wires, same miss, same spot. #4
Thank you

Thanks for the tips guys!
I am not able to look till tmrw due to some personal life things going on, but I will keep this in mind!
I will update as I move along when I get a chance too.
Thank you soo much for your helpful insight into this problem as I slowly get there!
I will also update what fixes it when i get to that point too. So I dont leave ya hanging wondering!
Again, I can't Thank You enough for your insight into this!

so i got around to the car today!
i swapped the coils around. and went from error P0304 - misfire # 4 to error P0300- random misfire "all coils and icm is brand new"
although i had time to do this i did not have time to trace wires and pull spark plug. i plan on doing this as soon as i possibly can.
Thank you!

i was just informed.
Owner put Heet gas additive in it during our cold snap. -49 degrees F
this was a few weeks back now.
personally i feel these things are snake oil. my 99 cavalier was fine during this snap, just hard to start that day :P
could this be a cause?

Hey Bud just saw your post had same problem it was my ignition control module that the coils sit on it was about 300 hundred bucks but pretty easy to change

I had the same issue but replaced the plugs, wires, fuel pump, ignition control module and coils. What finally fixed it was a new fuel filter.

The fuel pump was for the sender unit but if haven’t done that it’s a cheap try.