My car is doing the exact same thing. The soiund is coming from the engine. What exactly is piston slap, and how can it be fixed?

 

Piston slap is when the pistons comes too far up and hits the spark plug or the head. As for knock... the car is constantly advancing and retarding timing based on readings from its many sensors. As your RPMs go up, you have more and more timing advance to accomodate the faster moving piston. The idea is to get max power on the power stroke of the engine. When the engine is moving real fast the spark plug needs a head start on the piston... When it gets too much of a head start and combusts while the piston is still on the compression stroke you can imagine what happens.... It's very bad, the combustion is actually trying to push the rotating assembly the opposite way and since that is not going to happen, the pistons takes the brunt of it.

 

I thought that piston slap was when an engine with forged or hyperteunetic pistons was started when it was cold and the piston had actually "shrunk" in the bore. I have heard that this is normal for cars with these pistons in cold climates and when the engine temp heats up, the piston expanded and the slap went away. I know that if a piston was to hit a plug or the head, there would be a lot more wrong with it than piston slap.
Also, check the EGR valve. Sometimes those can become very noisy, and I know that when it's REALLY cold out, mine is quite loud.

 

Correct
Unless its INSANLY cold, a car should not do this. there is an issue with the piston tolerances on LS1 derived motors with this issue. Thank God mine doesn't have this problem.

Both are called knocks though
When it hits the heads, its called a timing/spark knock

 

Bad info here...

I can see 10* of knock and the car "sounds" normal. You need to get a scanner and see what is going on. That is the ONLY way to know for sure. If you use your ears you might hear something too late.

Ryan

 

yeah thats true, but I was just describing what it sounds like

you usually cant hear a few degrees of knocking-
but if you can HEAR it, its a pinging sound

 

What you meant to say was that when you hear knock, you will know it . Once you hear the pinging, you will know! You won't need a scanner to realize you need to back off until you can figure out the cause/solution.

However, after driving around with a scanmaster I don't think I could live very long with a modded car, not knowing what was going on in there.

Ryan

 

Did you solve the cause of the problem? If so, what was it? My 97 lt1 ws6 starts off with a rough idle and noisy (sounds like diesel) engine until it warms up (has always done this and has typical free mods). But lately, I've noticed a 20% drop in fuel economy, a major loss of power and the diesel sound when warm and under mild to hard acceleration. I've just switch gas brands and noticed a slight increase in power. Before, the engine was pulling back at 3500rpm, now it pulls back at about 4500rpm and the engine just seems to make a lot of noise. I drive easy. When running right, this car runs out beautifully and smoothly up to redline with a sweet engine sound. I'm really hoping it's trash gas.