Low compression in one or more cylinders can be caused by many possible failure modes. Here's a diagnostic procedure to isolate the culprit.

Could be caused by: 1. Cracked head. 2. Blown piston 3. Cracked Block 4. Blown head gasket 5. Broken valve component - spring, cracked seat, bent valve, bad lifter

Make an air fitting out of an old sparkplug with the guts removed. Remove the valve cover for that head. Check for a hung up valve by removing the rocker arms for both valves and observing the height of the valves - should be equal.

Insert the air fitting in the spark plug hole and add compressed air - the piston will be forced to the BDC position so watch your hands as the crank rotates.

Now listen for a loud hissing from one of the following: 1. Crank case 2. Intake manifold 3. Exhaust pipe 4. External cylinder head/block interface

Wherever the hissing is coming from will tell you where the compression is going.

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