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GMC V6 and V12 Engines Engine repair and rebuilding

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  #1  
Old May 26th, 2024, 08:43 PM
Prowbar Prowbar is offline
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Default Intermittent misfire diagnose and fix

Was chasing a misfire on the right cylinder bank for a while. Appeared and became more frequent over time, usually noticeable at idle.

Figured it was caused by ignition issues. Ended up replacing the spark plugs, didn't fix it.

Suspected a HEI module went out, which happened to me before, causing a misfire. Reverted the truck back to points, I like them better. the HEI didn't add anything for me, but that's just my opinion. Started moving everything back to how the factory did it, prefer it that way.
Anyway, misfire is still there with the points.

Did some thinking and realized it had to do with a vacuum leak, causing a mixture that is hard to ignite. Removed the valve cover and used a rag to cover the PCV valve. Misfire gone.

Ended up removing the valves and plugging off the ports. Running a open breather now, but will switch to a 460 Ford breather like sclor has posted, see link: https://6066gmcclub.com/showthread.php?t=51273&page=2
Post #15.

No more issues with the worn out PCV valves leaning out the air fuel mixture on 4 out of 6 cylinders.

Video showing the diagnose and fix:
https://youtu.be/EvM2PMCUji0

Last edited by Prowbar; May 27th, 2024 at 04:25 PM. Reason: typo
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Old May 26th, 2024, 08:47 PM
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FetchMeAPepsi FetchMeAPepsi is offline
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Default Re: Intermittent misfire diagnose and fix

Quote:
Originally Posted by Prowbar View Post
Removed the valve cover and used a rag to cover the PCV valve. Misfire gone.
That's the most satisfying sentence I've read all week! Good job tracking down the culprit!
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Old May 26th, 2024, 11:29 PM
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Default Re: Intermittent misfire diagnose and fix

I LIKE this post Prowbar!

Thanks for the diagnoses video and link to SCLOR's fix.

Much appreciated
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Old May 27th, 2024, 04:26 PM
Prowbar Prowbar is offline
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Default Re: Intermittent misfire diagnose and fix

Appreciate the replies guys. Hope this helps someone down the road!
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Old May 28th, 2024, 05:26 PM
Jim A Jim A is offline
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Default Re: Intermittent misfire diagnose and fix

No PCV valve in my '60, but I have found them to cause several different problems is several different vehicles. Always worth a look.
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