Compression Check: Testing and Inspection

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

Compression Gauge Check
1. Make sure oil in crankcase is of the correct viscosity and at proper level and battery is properly charged. Operate vehicle until engine is at normal operating temperature. Turn ignition switch to the OFF position and remove all spark plugs.
2. Set throttle plate in wide-open position.
3. Install a compression gauge such as Rotunda Air Pressurization Kit 303-F011 (014-00707) or equivalent in No. 1 cylinder.
4. Install auxiliary starter switch in starting circuit. With ignition switch OFF, using auxiliary starter switch, crank 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.
5. Repeat test on each cylinder, cranking the engine approximately the same number of compression strokes.

Compression Pressure Limit Chart - 3.0L (2V) And 3.0L (4V) Engines:

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 clean Motorcraft Super Premium SAE 5W-30 Motor Oil XO-5W30-QSP or equivalent meeting Ford specification WSS-M2C153-G on top of the pistons in the low-reading cylinders. Repeat compression pressure check on these cylinders.

Compression Readings-Interpreting
1. If compression improves considerably, piston rings are faulty.
2. If compression does not improve, valves are sticking or seating poorly.
3. If two adjacent cylinders indicate low compression pressures and squirting oil on pistons does not increase compression, the head gasket may be leaking between cylinders. Engine oil and/or coolant in cylinders could result.
Check the appropriate Compression Pressure Limit Chart when checking cylinder compression.

Example Readings
If, after checking the compression pressures in all cylinders, the highest reading was 1351 kPa (196 psi) and the lowest pressure reading was 1069 kPa (155 psi), the engine is within specification and the compression is satisfactory.