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P0304 Engine misfire, but only after heat soak, GM 2.2L 4-cyl

Greetings all, new member and first post…

Have a 96’ Chevy Cavalier with 21,000 original miles (inherited recently from family member and original owner).
When cold car starts immediately and runs perfectly if taken on a long or short ride. However once you turn the engine off and wait 10 or 15 minutes and then restart it, it runs rough and throws the P0304 code. If car started again 2 or 3 minutes after initial shutdown, it runs fine, only throws the code if it sits for 10 minutes or so!
So far I’ve replaced, to no avail;

  • Oxygen sensor #1
  • Spark plugs (all 4)
  • All spark plug wires
    Your thoughts and recommendations would be greatly appreciated.

The coil pack may be effected by heat.



If coil pack passes check compression


I always resort to unplugging injectors and or coil packs or individual wires, before pulling out the compression tester… But to each his own troubleshooting style…

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Thanks all, coil packs were next on my list. I’ll report back with results :oncoming_automobile:

What is a “pass” in your book?

I’ve seen plenty of coils pass any and all resistance tests, yet the misfires don’t go away until they’re replaced

I’ve even seen coil packs pass primary and secondary resistance tests, yet they’re actually cracked, if you clean them and look at them very carefully. And sometimes those cracks aren’t visible, until you physically remove the coil from the car and inspect the part on the bench

I’ve also seen misfire(s) caused by a “sloppy” fit . . . in regards to the spark plug wire attaching to the coil. In some cases, it’s the wires, and in other cases it’s the coil.

If there’s any corrosion on the coil tower, just replace the coil(s) and get it over with

Make sure you can get a solid engagement . . . a click you can feel and hear . . . when you attach the plug wires to the coil. Same thing goes for the other end, when you attach the wires to the plugs

it goes without saying to use high quality parts . . .

most of this comment . . . with the exception of my initial question to you . . . was really meant for @gebrown_166697 and not you

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Is this the “Quad 4” engine we are discussing ? If it is, the ignition systems on these engines have their own set of bugaboos and troubleshooting steps and the system is somewhat unique.

What vehicle are we discussing here? It matters greatly.

The Quad 4 was also known for headgasket issues. This could be the beginning of that issue as well. If it’s a Quad 4…

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Boy Howdy that surely does happen…

Yeah, most people don’t have the type of equipment needed to properly evaluate dielectric breakdown in a coil assembly. A hipot tester is more appropriate but even those may not spot early signs of failure at higher voltages than they can produce. A resistance test using a common DVM supplying around 9 volts won’t uncover early failure modes in the coil that can and will affect coil operation at full voltage.

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Okay folks I’m calling issue resolved. Had my local Firestone troubleshoot problem and they replaced a leaky fuel pressure regulator, no codes for three day now.

Thanks all for your imput :+1:

Good to know, add that to the list of possible problems…

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