Engine Compression Test. Pontiac Sun Runner - 4WD (Canada) 1998

For 1990-2009 cars

A compression test must be performed in order to determine the cylinder compression pressure. A compression test is used to diagnose the valve or pinion faults and poor engine operation. When checking the cylinder compression pressure the following must be present:

• | The throttle must be wide-open.

• | Remove all four of the spark plugs.

• | The battery must be at or near full charge.

Test each cylinder four times.

### Testing Procedures

#### Tools Required

J 39313 Spark Plug Port Adapter

Important: Be certain to clean any dirt or debris from around the spark plug holes prior to removing spark plugs.

1. Turn the ignition switch to LOCK.
2. Remove the four spark plugs.
3. Disconnect the distributor electrical connector.

Refer to Distributor Replacement in Engine Electrical.

4. Remove the F1 (15 amp) fuse from the fuse block.
5. Install a J 39313 and a compression gage into a spark plug hole.
6. Disengage the clutch on manual transmission equipped vehicles in order to lighten the starting load on the engine.
7. Depress the accelerator pedal to the to the floor for a wide open throttle.
8. Crank the engine, with a fully charged battery, in order to take four pressure readings.

• | Measure the highest pressure reading on the compression gage.

• | The standard compression pressure is 1,400 kPa (199 psi) at least 250 rpm.

• | The minimum allowable compression pressure is 1,200 kPa (170 psi) at least 250 rpm.

Important: The maximum allowable compression pressure difference between any two cylinders is 100 kPa (14.2 psi) at least 250 rpm.

9. Repeat the compression test procedures for the remaining three cylinders.
10. Remove the J 39313 and the compression gage from the engine.
11. Install all four spark plugs and connect the distributor electrical connector.

Refer to Distributor Replacement in Engine Electrical.

12. Install the F1 (15 amp) fuse to the fuse block.

#### Evaluation

• | Normal Operation

| The pistons, piston rings and valves are operating normally if the cylinder compression pressure increases quickly and evenly to the compression standard (1,400 kPa/199 psi at least 250 rpm).

• | Faulty Piston Rings

| The piston rings are probably at fault if the cylinder compression is low on the first stroke and tends too increase on following strokes, but does not reach the compression standard. The addition of oil should improve the compression pressure considerably.

• | Faulty Valves

| The valves are probably faulty if the cylinder compression is low on the first stroke and does not increase on following strokes. The addition of oil will not significantly improve the compression pressure.