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Piston Slap knock: Louder when engine/weather is cold and during lower RPMs. Then in some cases goes away. Non-detonation issue.
Carbon Knock: Louder when engine/weather is hot and during higher RPMs. Will not go away. Detonation issue. Higher-octane/slower burning fuel should solve.
Why then, Does GM refer to carbon build-up as a cold-start knocking issue in this TSB:http://www.expage.com/gmtsbs
Just seems to be contradictive.
GM, I agree. In todays engines, with the taper of the piston tops and shorter skirts, some normal slapping will occur up to 1-2 or even 3 minutes in extreme cold weather. But those aluminum pistons should expand quickly and quiet down. (my 5.7 goes on for 15-25 minutes--most people agree that's not normal but GM says, "yes").
If the pistons don't seat quickly, excessive blow-by will cause higher than normal air-pressure in the oil pan causing oil to be blown up into the valve cover and down from the top where it is burned in the combustion chamber. Higher oil "usage" from higher reving, higher compression engines might be normal, but oil "burning" from blow-by is not.
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