Jim Grant's Tech Tips
'99 Chevy
Silverado P/U, Engine Knock When Cold
Q:
My ‘99 Chevrolet Silverado knocks on a cold start. It has the 325
engine and now has 40k miles. It started at 5k and is getting worse. Chevy
repeatedly assures me this is normal. I don't believe them. The oil pressure
is up and the motor still knocks for about 30 seconds. When I give it
the gas it stops and starts again when I let off the gas. Once it is warmed
up there is no more knock.... Robert
A:
It almost sounds as if your vehicle’s engine has too much piston
skirt clearance. When a piston is not fitted correctly to a cylinder it
will produce a knocking sound that is most pronounced when the engine
is cold and will fade as the engine warms up. A piston with this condition
is not normal, and will only become more pronounced with age. If bad enough,
the piston skirt can break, which will result in an increase of noise
and oil consumption. Once again engine knock is not a normal condition.
GM’s 325 engine is a surprisingly gutsy and quiet. What is normal
is strong torque and performance unexpected of an engine of it’s
size.
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