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Can you name this knock?

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I have 99 GP GT 3800 NA. The motor has 86xxx miles and was meticulously maintained. One day the cat went out and was taken to the dealer. The car came back with a very noticeable knock. The dealer actually “rebuilt “ the lower end for some reason after bringing this to their attention. All I paid was time(6 months)and parts which spoke volumes about what had transpired while there as something clearly went wrong. I received the car back and now I’m dealing with another knock only on the top end. Took it to GM and of course all they said was “the top end has to be rebuilt but we can replace the motor”, absolutely no help identifying the issue at hand. 


I can’t find a reliable tech and need some real professional help which is why I’m here.


Any help is GREATLY APPRECIATED. Tia guys.


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I have named this knock "Earl".  As the song says, "Earl's gotta die."


Pull both valve covers, look for broken valve springs, damaged rockers, bent pushrods.  Might want to see if there's an exhaust leak at the manifold--loose bolts, cracked, etc. while you're in there.

Connect a vacuum gauge to manifold vacuum.  Ground the wire of a 12-volt test light.  Slide the pointy end of the test light between the plug wire and the rubber boot--DON'T DAMAGE THE WIRE (a dab of silicone grease on the test light probe helps.)  This will short out the spark while causing no damage to anything (if you're quick about it.  Short the spark too long, and you'll overheat the catalyst.)  See if the knock changes pitch as each cylinder is shorted out in turn.  Watch the vacuum gauge--each cylinder should drop the vacuum the same amount.  Cylinders that DON'T drop the vacuum when the spark is shorted are weak or dead.

Cranking compression test:  If all cylinders are NOT within 15% of each other, perform a cylinder leakdown test on the two lowest-pressure and the highest-pressure cylinders.

Edited by Schurkey

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