Mechanics

ENGINE KNOCK WHEN COLD

2005 Chevrolet Trailblazer • 6 cylinder 4WD Automatic • 76,000 miles

2005 trailblazer, 6 cylinder, 4x4, has a knocking noise when it is cold. Noise goes away after warm up. This started after I had the head replaced from overheating. Does anyone know what this could be?
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Wilsontr427
January 10, 2011.



First a little advice for the future. When an engine overheats and cause enough damage to blow a head gasket or cause you to replace an entire head, that engine should be rebuilt completely or replaced with another. It's sort of like giving a kidney transplant to an alcoholic.

Now for your present situation, I'd have to be sure your "knocking" sound is the same as my "knocking" sound.

What I call knocking is a term to describe "pre ignition". This is when there's a severe lean air/fuel ratio in the engine, or a hot spot that's causing the fuel mixture to ignite before the spark plug tells it to. (Thats a very simplified version)

This could quite possibly be your problem, but the computer would NO DOUBT see that and set a code. So do you have a check engine light on? If so, have the computer scanned.

Also, a lot of people hear a noise and call it "knocking", but what they're hearing is piston slap or a bad rod/main bearing. (The force of the piston is pushing the crankshaft against a worn bearing during the power stroke)

Everything I mentioned so far can be issues experienced with an engine after a severe overheating situation. With pre ignition, this can be solved by correcting whatever is causing the lean air/fuel mixture. With piston slap, you can try a heavy weight oil, but most likely you'll have to live with it unless you want to rebuild the motor. With the rod knock, make sure the oil filter isn't clogged, and maybe use heavy weight oil. You'll also have to live with it, but your engine won't live for long.

Ernest Clark
Feb 21, 2011.