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 the oil in the crankcase is of the correct viscosity and at the proper level. Make sure battery is properly charged. Operate the vehicle until the engine is at normal operating temperature. Turn the ignition switch OFF. Remove all spark plugs.
2. Install a compression gauge such as Compression Tester in the No. 1 cylinder.
Install an auxiliary starter switch in the starting circuit. Use the auxiliary starter switch, with the ignition switch off, to 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.
4. Repeat the test on each cylinder, cranking the engine approximately the same number of compression strokes.

TEST RESULTS





1. The indicated compression pressures are considered within specification if the lowest reading cylinder is within 75 percent of the highest reading. Refer to the Compression Pressure Limit Chart.
2. If one or more cylinders reads low. squirt approximately one tablespoon of clean engine oil meeting Ford specification WSS-M2C153-G on top of the pistons in the low-reading cylinders. Repeat the compression pressure check on these cylinders.

INTERPRETING COMPRESSION READINGS
1. If compression improves considerably, the piston rings are damaged.
2. If the compression does not improve, the valves are sticking or sealing improperly.
3. If two adjacent cylinders indicate low compression pressures and squirting oil on the piston does not increase the compression, the head gasket may be leaking between the cylinders. Engine oil and/or coolant in the cylinders could result from this condition. Use the Compression Pressure Limit Chart when checking the cylinder compression so that the lowest reading is within 75 percent of the highest reading.