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I have a 2009 Cobalt SD with 55000 miles. It is running very rough and knocking. Mechanic says compression is only at 50 (instead of 150) in two cylinders...and only fix is a new motor. Could this be caused by anything else?
 

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Piston rings, valve seals, or head gasket. Would need a leak down test to know what is for sure the cause.
 

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A one tooth timing chain jump would do these very symptom. Rough idle, knocking and very low power.
 

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That would cause low compression on just 2 cylinders? I would think that would be all 4
 

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Nope just 2 and it would be the other if it jumped the other way. 1-4 or 2-3, this is if it jumped at the crank if only one cam jumped different symptoms again.
 
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