Compression Check: Testing and Inspection

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

General Remarks

Note: Removing fuses and disconnecting electrical components causes the Powertrain Control Module (PCM) to log an error message. After the measurements have been carried out this error message should be cleared from memory by connecting to PDU.

Note: Only check the compression pressure with the valves set to the prescribed clearance (if this can be adjusted).

The compression pressure should be checked with the engine at operating temperature.

Check The Compression Pressure

WARNING: On manual transmissions shift the transmission into neutral. On automatic transmission vehicles, select "P". Failure to follow these instructions may result in personal injury.

1. Remove the fuel pump relay.
2. Start the engine-the engine will start, run for a few seconds then stall.
3. Remove the spark plugs.
4. Install the compression tester.
5. Install an auxiliary starter switch in the starting circuit. With the ignition switch OFF, using the auxiliary starter switch, crank the engine a minimum of five compression strokes and record the highest reading. Note the approximate number of compression strokes required to obtain the highest reading.
6. Repeat the test on each cylinder, cranking the engine approximately the same number of compression strokes.
7. Install the components in reverse order, observing the specified tightening torques.
8. Reset the PCM fault memory.

Interpretation of the Results
The indicated compression pressure are considered within specification if the lowest reading cylinder is within 75% of the highest reading.

CAUTION: If engine oil is sprayed into the combustion chamber, after carrying out the measurement run the engine at 2000 rpm for about 15 minutes, in order to burn the oil and prevent damage to the catalytic converter.

If the measurement on one or more cylinders is much lower than the specified value, spray some engine oil into the combustion chamber and repeat the compression measurement.

If the reading greatly improves, the piston rings are damaged.

If the reading stays the same, the cause is either damaged valve seats or valve stem seals.

If the measurements for two cylinders next to each other arc both too low then it is very likely that the cylinder head gasket between them is burnt through. This can also be recognized by traces of engine oil in the coolant and/or coolant in the engine oil.