Cylinder Compression Testing

A wet compression test is performed if a cylinder’s pressure is below specifications. This test indicates whether the valves, head gasket, or the piston rings are at fault. This is done by injecting a small amount of oil, usually about a tablespoon, into the cylinder and rotating the crankshaft a number of times, about six turns of so. If the compression gauge indicates an increase after adding the oil, the piston rings are worn. If the readings change little or stay the same, the head gasket, a valve, or valve seat is faulty. To repair this condition, the cylinder head is removed and the faulty valve or seat repaired.

Compression test results.

Typical wet compression test:

1. Warm the engine to operating temperature and then let it sit for 10 or 15 minutes.

2. Remove all spark plugs and open the throttle. Keep the plugs in order by cylinder.

3. Disable ignition and fuel.

4. Test each cylinder and record results before adding oil .

5. Add 1 tablespoon of oil to the cylinder with 85 lbs, rotate the crankshaft 6 times, and record the readings.

6. If the reading increased, the engine needs piston rings, if not, suspect the cylinder’s intake and exhaust valves.