5.3 vortec heavy misfire under load

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Originally Posted By: meep
There's another possibility we haven't covered--- predet due to carbon buildup or a hotspot such as a poorly seated exh valve. That would do exactly whats listed above and could be informally tested by a tank of 93 octane....


I have seen lots of this (carbon knock) in our fleet trucks when they get very high mileage. A dose of kreen in the gas clears it right up. Most times they make a noise similar to piston slap when first started cold when it builds up a bit...
 
Originally Posted By: SteveSRT8
Originally Posted By: meep
There's another possibility we haven't covered--- predet due to carbon buildup or a hotspot such as a poorly seated exh valve. That would do exactly whats listed above and could be informally tested by a tank of 93 octane....


I have seen lots of this (carbon knock) in our fleet trucks when they get very high mileage. A dose of kreen in the gas clears it right up. Most times they make a noise similar to piston slap when first started cold when it builds up a bit...

Yeah my truck had it when I got it. Didn't get driven much or hard.
I ran a couple Seafoam treatments and it cleared it up. I think the heavy towing helped too.
 
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