question about piston slap

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Feb 21, 2004
So. Cal.
it is my understanding that piston slap occurs mainly at cold start up,or warming the engine up.what about at operating temperature? idle? would it be heard as a light knock or diesel like rattle? just curious..also does the bypass valve in the oil filter make a distinct sound if malfunctioning?
moontan, do a search in this subforum and the mechanical tips & tricks subforum using the phrase "piston slap."

You should see a lot of what has already been said.

--- Bror Jace
If you own a GM brand pickup (Chevy Silverado or GMC Sierra) with Vortec 4.8, 5.3, 6.0 or God Help You 8.1 litre V8, you will be well aware of "cold start knock" or "piston slap."

There appear to be many variables involved and what cures the problem for one person will not work for another.

For example, in winter temps colder than -18 C or 0 F, with the factory-fill 5W-30, my truck made the knock-knock noise for up to 90 secs. Changing to Mobil 1 0W-30 eliminated the noise.

My Vortec 5.3 motor has never made the knock noise in summer.

There are numerous TSB's for replacement pistons, bad o-rings in the oiling system, carbon build-up, etc etc.

Some of those Vortec motors knock so loud on startup you'd swear it was an older Cummins diesel.

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