Compression Test

DESCRIPTION
A compression test is one of the best tools to assist a technician in properly diagnosing valve or piston faults and poor engine operation. To accurately determine the compression pressure, the throttle must be wide open, all spark plugs must be removed and the battery at or near full charge. Test each cylinder four times.

TESTING PROCEDURES

- Tool Required:
- J 22794 Spark Plug Port Adapter, Or Equivalent

NOTE: Be sure to remove any dirt and debris from around spark plug holes prior to removing spark plugs.

1. Stop engine.
2. Remove all four spark plugs and disconnect secondary (coil) wire from ignition coil. Refer to Ignition System.
3. Install a J 22794 and a compression gage into a - spark plug hole.
4. Disengage the clutch (manual transaxle equipped vehicles) to lighten starting load on the engine and depress the accelerator pedal to the floor (wide-open throttle).
5. Crank engine (with a fully charged battery) to take four pressure readings. Obtain an engine speed of at least 250 rpm or more. Record all four readings.
Measure
- The highest pressure reading on the compression gage.
- Standard compression pressure is 1,320 kPa (191 psi) at 250 rpm.
- Minimum allowable compression pressure is 981 kPa (142 psi) at 250 rpm.
6. Repeat compression test procedures for remaining three cylinders. Record all four readings for each cylinder.

NOTE: The maximum allowable compression pressure difference between any two cylinders is 98 kPa (14 psi) at 250 rpm.

7. Remove the J 22794 spark plug port adapter and compression gage from the engine.
8. Install all spark plugs and connect the secondary (coil) wire to the ignition coil. Refer to Ignition System.

EVALUATING TEST RESULTS
1. Normal operation-If the cylinder compression pressure builds up quickly and evenly to the compression specification (1,320 kPa/191 psi at 250 rpm), the pistons, pistons rings and valves are operating normally.
2. Piston rings faulty-If the cylinder compression is low on the first stroke and tends to build up on following strokes (but does not reach the compression standard), the probable cause is the piston rings are faulty. The addition of a small amount of oil through the spark plug port should improve the compression pressure considerably.
3. Valves faulty-If the cylinder compression is low on the first stroke and does not build up on following strokes, the probable cause is that the valves, valve seats or a gasket are faulty. The addition of oil does not significantly improve the compression pressure.
4. Faulty gasket-If the cylinder compression on both of any two consecutive cylinders (1-2, 2-3 or 3-4) is low and does not reach specification, the probable cause is a blown head gasket between the cylinders, allowing compression to leak between cylinders.