Engine knocking when cold

My 2001 GMC Sierra with 5.3 engine has a knocking noise when the engine is started cold & lasts for about 15 seconds then stops. I was sure it was a heat riser, but found out that they don’t put heat risers in the exhaust anymore! Has been doing this several months with no other problems! I also get 21 mpg on trips here in the Northeast.

It may be piston slap that was in some GM V8 engines of that vintage.

I don’t know what your manual suggests, but you may want to use a lighter weight oil in the winter. I always use 10W30 in the winter and 20W50 in the summer in my V-8s.

So what do you do about piston slap?

There’s nothing that can be done about piston slap except to replace the engine. Which GM did for some customers who complained that they shouldn’t have an engine in a $40,000 vehicle make that kind of noise.

Piston slap occurs when the skirt of the piston slaps the cylinder wall as the piston moves up the cylinder. As the engine heats up the pistons expand and fit tighter in cylinders and the piston slap stops. Piston slap is not damaging to the engine. Just embarassing as hell to have a vehicle that cost that much to have an engine make that kind of noise.

And by the way, you don’t want to use a 10W30 oil in the winter. But instead use a 5W30 oil in the winter. The 5W and 10W reflect the Winter weight of the oil. So a 5W30 oil will flow much faster to protect vital engine components during a cold start better than a 10W30 oil will.


The only oil I use year round is 5w30! I really appreciate the info, and you’re right, it is embarassing!

A Google of “GM piston slap” will turn up tons of info including whether your engine is one of those proe to the problem.