I have a 2000 GMC jimmy with 85,000 miles. Which has started knocking when I first start it and coninues to knock until the engine reaches 210 degrees.

Any ideas?

The GM engines of this era were notorious for piston slap.

Piston slap occurs when the skirt of the piston slaps the bottom of the cylinder bore as it moves up the bore. As the engine heats up, the pistons expand slightly and that’s when the noise stops. This is an example of what happens when manufacturing tolerences are loosened up slightly. And this noise could start happening with just a couple thousand miles on the engine, or after many thousands of miles were put on the engine.

This was such a problem with these engines that owners were complaining to GM that a new vehicle shouldn’t make this type of noise. And GM installed new engines in these vehicles, depending on the mileage on the vehicle.

Piston slap isn’t harmful. It’s just embarrasing as hell as you’re driving along while the engine is making this noise.