Compression Check: Testing and Inspection

AllData Editors Note - Ford does not provide a cranking pressure specification, only an acceptable range.

Before performing the compression test, check the battery. The battery must be fully charged, in good condition, and properly connected. If the battery is not fully charged, recharge or replace it.

Compression Gauge Check






1. Run the engine until normal operating temperature is reached.
2. Turn the ignition switch OFF.
3. Release the fuel pressure using the following procedure:
a. Remove the 15A FUEL PUMP fuse from the interior fuse junction panel.
b. Start the engine and allow the engine to idle.
c. Let the engine stall, and crank the engine three more times to ensure fuel is out of the system. Turn the ignition switch OFF.
4. Remove the spark plugs.
5. Disconnect the electrical connector from the ignition coil.






6. Install Rotunda Quick Disconnect Compression Tester 134-R0212 or equivalent tightly into the spark plug hole.
7. Depress and hold the accelerator to Wide Open Throttle (WOT) and crank the engine.
8. Normally, the first compression stroke will run the gauge indicator needle a considerable distance up the scale. Succeeding strokes will raise it more until the highest level is shown (this will require approximately four to five compression strokes). Record the highest reading.

Test Results
1. Repeat steps 6-8 for each cylinder and compare the readings with the following specifications.
- Standard compression is 1200 kPa (175 psi)
- Minimum compression is 885 kPa (130 psi)
- Maximum difference between cylinders should not exceed 100 kPa (14 psi)

NOTE: Remember that the engine must be at operating temperature and the throttle valve must be held open during the compression test.

Compression Readings - Interpreting
Variations between cylinders will have a greater effect upon engine performance than overall readings that are equal between cylinders but slightly below specifications.

When taking compression readings, watch the action of the gauge needle. When it raises only a small amount on the first stroke, a little more on succeeding strokes, and results in a low reading, burned, sticky, or warped valve(s) are indicated.

A low buildup on the first stroke with a gradual buildup on succeeding strokes leading to a moderate reading can mean worn, stuck, or scored piston rings.

If two adjacent cylinders are low, a blown head gasket or warped cylinder head-to-cylinder block surface could be responsible.

Add one teaspoon of heavy (SAE 30 minimum) engine oil to a cylinder with a low reading. Insert the compression gauge and recheck the cylinder. Crank the engine for a few extra compression strokes and watch the gauge. If the compression goes up 10 percent or more, worn or damaged piston rings are indicated. If the addition of the oil produces no significant change, valve trouble, a broken piston, or a blown head gasket is probably causing the low reading.

If the compression pressure exceeds specifications, there is a buildup of carbon on the head of the piston and on the combustion chamber walls. If pinging cannot be stopped by retarding the timing, or by switching to a higher octane gasoline then carbon buildup may be indicated.

Example Readings






The Cylinder Pressure Variation Chart gives the allowable variation in cylinder pressure. If the maximum and minimum readings are not within specification further diagnosis is necessary.