Compression Test

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

Compression Test
1. Make sure oil in crankcase is of the correct viscosity and at the proper level and that the battery is properly charged. Operate the vehicle until the engine is at normal operating temperature. Turn the ignition switch to the OFF position, then remove the spark plugs.
2. Place the transaxle in PARK (A/T) or NEUTRAL (M/T) and apply the parking brake.
3. Block throttle plate in the wide-open position.
4. Install a compression gauge such as Compression Tester in the No. 1 cylinder spark plug hole.
5. Install an auxiliary starter switch in the starting circuit. With the ignition switch in the OFF position, and 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.

Compression Pressure Limit Chart:

Test Results
The indicated compression pressures are considered within specification if the lowest reading cylinder is within 75 percent of the highest reading.
If one or more cylinders reads low, squirt approximately one tablespoon of Super Premium SAE 5W30 Motor Oil X0-5W30-QSP or equivalent meeting Ford specification WSS-M2C153-G on top of the piston(s) in the low-reading cylinder(s). Repeat the compression pressure check on these cylinders.

Interpreting Compression Readings
1. If compression improves considerably, piston rings require service.
2. If compression does not improve, valves are sticking or seating improperly.
3. If two adjacent cylinders indicate low compression pressures and squirting oil on piston does not increase compression, the head gasket may be leaking between cylinders. Engine oil and/or coolant in the cylinders could result from this condition.

Use the Compression Pressure Limit Chart when comparing cylinder compression readings to determine if the lowest reading is within 75 percent of the highest reading.