Hey all. I'm new in here but not to LS1tech. I've got a 99 Silverado Z71 w/ the 5.3. Yesterday a friend of mine and I put a Flowmaster on my truck and when we started it back up it had a knocking sound. It sounded just like a bad rod bearing. You had to give it a little throttle and when it came back down you could hear it. It wasn't doing it while idling though. I drove it down the road maybe half a mile and got back and it stopped. Has anyone heard of the 5.3's making this kind of noise? Pushrod? Other than the Flow it's stock with 99K on it. Any help is greatly appreciated.

 

Probalby piston slap (if you are sure it isn't the exhaust againt the frame since that is what you worked on). All gen3 engines are prone to piston slap. The piston basically wabbles in the cyl when cold and when it heats up and swells it stops.

 

did the motor stop by it self r did u stop itwere did you put the ground if u put it were it may have going through the motorit may have messed up a bearing.i would check what u worked on last

 

I'm familiar with piston slap. That's not it though. It's still running fine, it's just not making the noise anymore. Oil pressure is good too. 40- about 80 psi.

 

It never stopped running. Only stopped making the noise. By ground, do you mean the welder? It was grounded to the muffler itself.

 

My 02 6.0 slaps when cold then it goes away. No worries

 

They all knock when cold, if it don't, it will.