My brother brought me his car today to figure out engine miss and fuel smell. Checked coils found one that seemed to be weak on 1 cylinder swapped it out with another coil , fixed a small hole in one of the main vacuum lines , checked the fuel regulator (replaced recently) cleaned the MAF and the air charge temp sender. Took for ride and the intermittent miss is still there, act's like a bad coil and when at speeds it acts as the torque converter is going in and out of lock up. No codes coming up. Local store cant test the ignition control module. Am I right in thinking the ICM or does anyone have other idea's ? Any back yard way to test module ? Only happens after engine is at operating temps.

 

Yeap... Been having this problem (not the intermittent miss) for FOREVER now....... PLEASE update me if you find anything out....

 

Maybe you have a U Pull it salvage yard you could get one at and put it on to see if that is the culprit, might be just as bad but then again you might get a good one too?

 

The ICM can definitely cause misfires. Typically the misfire throws a code. The last 3800 I messed with had a misfire and code P0300. The scan tool I was using was able to tell me the misfire history. I found it on a specific cylinder. Thought it was the coil, so I swapped with a known good spare, no change. I was pretty certain mine was an ICM issue, but I did not have a spare to throw at it. The test I used was a Harbor Freight inline spark tester (flashes a light when it gets spark). On the position with the issue, the light flashed but was VERY dim. In other positions, nice and strong, swapped the coil with another one on the car that I tested with the light as good, dim light did NOT follow the coil. This left only the ICM at that point. Swapped the ICM, all good.
And Autozone was able to test the ICM and it passed the test. I think I see a fault in how they tested it, but that's another conversation.

If you can get a scan tool that provides the misfire history stored in the PCM, that could uncover something. My other thought is perhaps something has failed that the car does not detect as a failure, resulting in the PCM making the engine mis-behave, but no code since the PCM believes it is doing what is right. If that is the case, this could get painful to trace down.

 

We tried what ChibiBlackSheep suggested by unplugging the MAF in another post https://montecarloforum.com/forum/mo...ctuates-59714/ and car seems to be running fine with no issues besides a small rpm drop at idle. Said it started running bad as soon as he pluged it back in and check engine light came on . Something with the heated O2 not sure he has my code reader. Might have to check into misfire if he didn't erase codes. Any other suggestion's would be cool . Will keep posted if I figure something out

 

If you unplugged the MAF and things appear to be working, that really points pretty strong to a failed MAF. When you unplug the MAF, it forces the PCM to run in a different mode (some call it "limp home" mode). I believe it actually places the PCM in Speed Density/VE mode.

 

Thanks Jason , I didn't hear back from my brother yesterday but I gave him my code scanner and tamper proof torx sockets to remove it. Working out of town this week ill let you know what we figure out . Appreciate the help.

 

I would really suspect the MAF if unplugging it had the car drive fine. However it does work in conjunction with the oxygen sensor to get the A/F ratio in check.

 

I also passed info along about disconnecting battery for a hour to reset limp mode.as you suggested in other post ChibiBlackSheep. Ill keep posted on outcom. Otherwise he'll use my daily driver for his test parts.

 

Don't trust AutoZone's testing for anything. My battery still has their sticker on it proclaiming it bad. Needless to say, the actual culprit (the alternator) was purchased elsewhere.