1996 Chevy Astro Knocking Noises

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Noises problem
1996 Chevy Astro 6 cyl Two Wheel Drive Automatic

I recently started to hear a knocking noise coming from my motor. I was told that it could be the connecting rod bearings or something in or around the crankshaft. Is either of these answers true?

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have the same problem?
Tuesday, April 6th, 2010 AT 12:57 PM

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Absolutely. Connecting rod bearings have a very specific clearance, or spacing, usually around.004". When they wear, they hammer against the crankshaft. Oil pressure will start to go down which will lead to accelerated wear of the other bearings. If caught early enough, the engine can be rebuilt and more of the parts can be reused. When the damage is more severe, the crankshaft and some of the connecting rods will also have to be replaced.

Your mechanic should be able to tell the source of the knock from the sound and by performing some tests or a visual inspection. Other potential causes include a broken hub on the vibration damper, a severely over-tightened serpentine belt, and a cracked flex plate at the back of the engine.

A cracked hub on the vibration damper, (harmonic balancer), will change sound when the serpentine belt is removed. The large bolt on the front of the crankshaft could also be loose.

An over-tightened belt pulls up on the snout of the crankshaft, bending it a couple thousandths of an inch. When one of the front cylinders fires, the force bends the crankshaft snout down causing a knocking noise when it hits the other side of the front crankshaft bearing.

A connecting rod knock will change when the spark plug is shorted out on the affected cylinder.


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Tuesday, April 6th, 2010 AT 3:26 PM

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