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| GMC V6 and V12 Engines Engine repair and rebuilding |
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#1
May 26th, 2024, 09:43 PM
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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 05:25 PM. Reason: typo 
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#2
May 26th, 2024, 09:47 PM
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Re: Intermittent misfire diagnose and fix
Quote:
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#3
May 27th, 2024, 12:29 AM
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Re: Intermittent misfire diagnose and fix
I LIKE this post Prowbar!
Thanks for the diagnoses video and link to SCLOR's fix. Much appreciated 
__________________
1961 GMC Suburban 305A Overdrive 1962 GMC Utility 305D
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#4
May 27th, 2024, 05:26 PM
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Re: Intermittent misfire diagnose and fix
Appreciate the replies guys. Hope this helps someone down the road!
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#5
May 28th, 2024, 06:26 PM
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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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