What is causing loss of compression?

Loss of compression can be due to old engine or problem in cylinder head gasket causing leaks.

If you don't know what to do consult or look for professional mechanic.

Recommendation: Replace engine if you over-used it. Like reaching high mileage over 100,000 or 200,000 or 300,000 miles.

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The engines that i typically see with low or uneven compression have burned or warped valves. Occasionally the rings are bad but that is unusual. The definitive test is to run a cylinder leak down test. You hook an air compressor up to a cylinder with a special tool . If air leaks out of the tailpipe then it is a bad exhaust valve. If air comes out of the Air filter snoot then it is a bad intake valve. And if air comes out of the Valve cover oil fill cap then it is the rings