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Can a mechanical problem, valves etc.. set code P0300 (random misfire)? The only ignition component I have not replaced is the ignition control module but I still have the misfire, Cad dealer suggested valves over the phone could this be??


Spark plugs

Plug wires

All coiles (4)

swapped injectors but cylender #5 remains the only misfire

via scantool

Thanks for any help,

Chris L.

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  • 2 weeks later...

A older friend of mine recently purchased a 1998 4.6L STS 46k miles. Today he was very worried because the check engine light came on and we was supposed to be driving from Northern Illinois (Elgin) to Tampa, Florida today. I pulled the code P0300 random misfire. The engine was warm when the random misfire occured under normal driving conditions.

I searched the archives for more information. I cleared the code. His engine ran rough for a short period of time last night and that was enough to set the code.

My current thinking is that he has only owned the car a few weeks and that it was prevoiusly babied. I instructed him on the usage of WOT's. I thought that it's possible the random misfire was caused by some carbon breaking loose causing a valve to remain slightly open. I would certainly be interested in knowing if there are anymore thoughts regarding the setting of the P0300 code and what it took to resolve the problem. Thanks...

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I am pretty sure a P0300 can be set by a mechanical problem. A few weeks ago a coworker had a very bad "misfire". This was on a Jeep (OBDII). I brought my code reader in and pluged in to the car and got a P0301 (misfire no. 1 cylinder). He went home and the next day changed the plugs, wires, distributor cap & rotor. It was still running rough and missing. I then checked, as best I could without a noid light, the fuel injector and it appeared to be good but he replaced it anyway. Still no improvement, so he took it to a local shop. They did a compression check and no. 1 was only about 60 psi. I told him we'd probably have to pull the valve cover to check for a broken spring or something mechanical. In the mean time he put some type of oil addative in the crankcase and gas tank as a last ditch effort ( Seafoam I think). I'll be damned if that engine didn't smooth out and has been running smooth for the last 3 weeks or so. So the moral of this long story is that it can be set by a mechanical problem and sometimes some additives do work. I don't want to start a big debate about additives, but I have to admit that when it comes to that subject I am pretty much in Guru's camp and consider most as "snake oil". I'd have bet against it but you can't argue with success.

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When I purchased my '94 (7 years ago), it had a misfire at low rpm. No codes set.

A compression test verified that #5 cyl had 55 lbs compression.

I tried all the WOT/combustion cleaner remedies, plugs, coils etc.

End result was pulling the head. It was a burned exhaust valve.

I replaced both exhaust valves in that cyl.

Replaced the head, (without timesert), and a year later, removed the head again for three pulled head bolt threads.

I used timeserts in all holes and it has been working great since.

The question is, have you done a compression test on that cyl ?


2008 STS V8
2016 Colorado Z71
1970 Corvette LT-1 Coupe

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