I am hoping that someone can offer some help with a cold start misfire that has recently developed on my 2004 Monte Carlo. It has a 3.4L engine with 140,000 miles. The misfire first manifested itself first thing in the morning with no problems for the the rest of the day. According to my scan tool, #1 cylinder is the culprit with #4 missing occasionally. The engine would only misfire for a maximum of 20 seconds at it worst, most generally only 5-10 seconds. Then it would would great, no misfire. No codes are being set.

The first thing I tried was to swap the entire coil pack assembly from another Monte Carlo that was running perfectly. The misfire went away for about a week. Again, #1 cylinder.
So I replaced the plugs, wires and #1/#4 coil all with AC Delco parts. Again the misfire disappeared, for a day. Then it came back, #1 cylinder.
I've checked all electrical connections and checked for vacuum leaks. I have not found anything obvious. But at least now the miss is confined to #1 cylinder, at least according to my scanner. I have no other way to check because the miss does not last long enough for conventional misfire checks.
One thing that I have noted is that if I rev the engine past 1500 RPM, the miss goes away sooner.
At this point, I am thinking that it is one of three culprits - either one of the two crank sensors or the PCM.
Anybody out there have any experience with this? Any ideas? Thanks for your help.

 

Check fueling. The injector could be pulsing poorly.

 

My apologies. I forgot to mention that I had replaced the injectors, AC Delco again. I figured with 140,000 miles it wouldn't hurt. Also, scanner shows no problems with injectors circuits. Problem is still present.

 

Have the plugs been changed? If so, are you running BOSCH plugs (if the answer is YES, remove the BOSCH plugs, switch to AC Delco or NGK).

Since the scan tool is reading mis-fires on two cylinders, I first would check if they share a coil pack (sounds like you covered that). Next, pull the ICM out, head to the parts store, have them test it (they can bench test the ICM with no problem). If those two items check out, go to basic maintenance:
- Get all new plugs/wires (if they are new, as stated first thing, make sure they are NOT BOSCH plugs). If they are new, might be worth pulling the plug and checking it out.
- Inspect/replace air and fuel filters

 

Yes the plugs have been changed as were wires. AC Delco replaced by AC Delco. I swapped the coil pack (coils, ICM and bracket) from another car which had no misfires which should tell if it's a coil or ICM. The car ran fine for about 5 days and then started missing again. The original coil pack is still on the other car and it's never missed a beat which tells me that it is not a coil or ICM. I did put in a new coil for #1/#4 cylinder but this didn't help. Air & fuel filters are new. At this time, misfire seems to be confined to #1 cylinder and only for 5-10 seconds on start up.

 

Follow the wires for the mis-firing cylinders. Check the sheathing on them for pre-mature wearing (some times caused by resting incorrectly on hot metal surfaces). If the sheathing is damaged in ANY way, this can result in arcing thus causing a mis-fire.

Arcing can sometimes be found by running the car at night, let it idle and mist water from a spray bottle in the area.

Just another thought.

 

i went threw this as well i had to replace the plugs to the coil pack ironically. when i replaced the plugs it went away for a little then came back. then i replaced the wires did the same went down to two cylenders then i replaced the coil pack and now no miss fire. but i would change out the plugs first and run a fuel treatment for a little. if it is still there it might be the wires or the coil pack. the wires are cheaper than the coil pack i would replace them. then go down the list. to make sure you have spark also check out to see if there is voltage going to the injectors as well.if there is still a missfire.

 

I re-read the original posting. Your symptoms almost match what my '94 Grand Am (3100, older version of your 3400 engine) had at one point.
It had a rough idle when started after sitting over night. After a few seconds or to the first stop sign it went away. The '94 I don't think can throw a code or tell what cylinder was mis-firing. I excused it at the time as the car had like 160,000ish miles on it.

Around 175-180,000 miles, I began having other issues (temp gauge running "hot", coolant system getting air pockets frequently AND becoming over pressurized). All these issues came down to a failed head gasket. My air pockets and over pressure in the cooling system was compression gases (I believe from a total of 2 cylinders). The rough idle at cold start was a result of a low amount of coolant forced into a couple cylinders while the car was sitting over night (and the presure went down in the cooling system).
The tell-tale sign at the end of my issues was clouds of white smoke when the car was first started.

I am NOT saying your issue is a head gasket, but it does have items that could suggest it. Do other diagnostics before coming to that conclusion. To my knowledge, the best way to confirm if it is or is not a head gasket, perform a compression test.

 

Maniac,

You hit the nail on the head. When the engine first started missing, I told my wife it was most likely the head gasket starting to go. But when the I swapped the coil packs and the problem went away for a few days, that made me think it was fuel/spark related. Also, the #1 plug was always dry when I pulled it out.
Finally, one morning I started thinking that if coolant only got into the cylinder after it sat, then the plug wouldn't be wet. So I decided to start it, run it for a couple seconds and shut it down while it was still missing. (Ironically when I talked to the local GM mechanic, he suggested the same thing.) So I did exactly that and the plug was definitely wet with antifreeze. I am now in the process of replacing the head gaskets. I guess I should have followed my first diagnosis.
Thanks everyone for your postings.

 

That sucks, but I'm glad you found it!! God only knows how long mine was bad before that puzzle came together for me. BUT, car runs great.

When you do this, get the Fel-Pro aluminum LIM gaskets (you MUST change the LIM gaskets, get the "worry free" ones).

Best of luck and again, glad you finally have an answer (even though it's a crappy answer, it's better then pulling your hair out trying to diagnosis).