GM Piston Slap website - Carbon buildup

[Trippers99hoe]

"Piston slap" usually describes pistons slapping the sides of the cylinders.

I would call this carbon knock, when the carbon builds up on the top of the piston and head, and the piston--> carbon <--head all touch ever so slightly causing noise.

When I worked for Mazda, the 929 had a recall on this. I tore down one of the first engines in 93-94.

Not sure if this is a good idea now,But in 1986 I worked at a Pontiac/Jeep dealer.We had several jeep products come in with Carbon buildup/knock
The service manager would rev the engine to about 2200rpm and pour a half glass of water slowly in the carb......it worked.

It was a long time ago but that is how I remember it........My dad used to do the same thing with ATF on old chevys.

 

[SunlitComet]

It works if done properly. It is basically steam cleaning the inside of engine.

 

[gpracer1]

Not sure if this is a good idea now,But in 1986 I worked at a Pontiac/Jeep dealer.We had several jeep products come in with Carbon buildup/knock
The service manager would rev the engine to about 2200rpm and pour a half glass of water slowly in the carb......it worked.

It was a long time ago but that is how I remember it........My dad used to do the same thing with ATF on old chevys.

I have done the same many times.
I love it when people tell you it will hydrolock or some other crap, then watch as I pour half a gallon right in at 4000 rpms, and I really mean pour.