Compression Check: Testing and Inspection

1. Disconnect the primary lead from the distributor or ignition coil.

2. Remove all spark plugs.

3. Block the throttle plate and choke plate (if used) wide open.

4. Make sure the battery is fully charged.

5. Starting with the compression gage at zero, crank the engine through four compression strokes (four "puffs").

6. Make the compression check at each cylinder and record each reading.

7. If some cylinders have low compression, inject about 15 ml. (one tablespoon or about 3 squirts from a pump type oil can) of engine oil into the combustion chamber through the spark plug hole.

8. Minimum compression recorded in any one cylinder should not be less than 70 per cent of highest cylinder, and no cylinder should read less than 690 kPa (100 psi). For example, if the highest pressure in any one cylinder is 1035 kPa (150 psi), the lowest allowable pressure for any other cylinder would be 725 kPa (105 psi). (1035 x 70% = 725) (150 x 700/0 = 105).

^ Normal - Compression builds up quickly and evenly to specified compression on each cylinder.

^ Piston Rings Leaking - compression low on first stroke tends to build up on following strokes but does not reach normal. Improves considerably with addition of oil.

^ Valves Leaking - Low on first stroke. Does not tend to build up on following strokes. Does not improve much with addition of oil.

^ If two adjacent cylinders have lower than normal compression, and injecting oil into cylinders does not increase the compression, the cause may be a head gasket leak between the cylinders.