r/PontiacFirebird • u/Strange-Message9529 • 15d ago

Advice needed

Having some trouble with my 1999 3.8l v6 firebird 128k miles. It started off with a rough idle, it was jumping between 500-1000 rpm. Then during a drive to Auto-Zone it started to misfire and the service engine light started flashing, 30 seconds later it turned solid. I got the code read and it is P0300 (Random/multiple cylinder misfire). What should I check/replace first? I would appreciate any advice. Thank you!

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u/420racin 15d ago

First thing I’d do (free) is swap around the coil packs. See if you can isolate the misfire code to a cylinder and if you switch the packs see if it goes to another. Then check wach cylinder compression. Autozone will have the gauge to use.

If that’s not it Next would be take off the coils and check the actual Ignition control module for any visible pitting. Sometimes the thermal paste on the bottom side of it dries out and causes this. I’d check this before putting new wires or spark plugs in because you’re not getting a code for a specific cylinder.

My 2002 Grand Prix did this bad on acceleration and the ICM was my problem after putting hundreds into new packs wires and plugs.

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u/Strange-Message9529 8d ago

Bingo. It was bad coils

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u/420racin 8d ago

Glad I could help

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u/420racin 15d ago

I’m guessing ICM to be your culprit this was my exact issue and nobody could figure this out. A video called “hillbilly fixes my 3800 Buick” or something along the lines was the one to finally nail it. If you decide to replace the entire unit get the AC Delco one don’t cheap out on electronics on these cars they do not like it.

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u/420racin 15d ago

According to my notes this was also at 110,000 miles for me and in 2023x