Compression Check: Testing and Inspection

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

Compression Test
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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, then remove all spark plugs .

2. Set throttle plate in wide-open position.

3. Install a compression gauge such as Rotunda Compression Tester 059-00009 or equivalent in No. 1 cylinder.

4. Install auxiliary starter switch in starting circuit. With ignition switch OFF, and using auxiliary starter switch, crank engine at least 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 the same number of compression strokes.

Test Conclusion

The indicated compression pressures are considered within specification if the lowest reading cylinder is within 75 percent of the highest. Refer to the following chart.






If one or more cylinders reads low, squirt approximately one tablespoon of XO-20W30-QR or equivalent engine oil meeting Ford specification ESR-M2C179-A on top of the pistons in the low-reading cylinders. Repeat compression pressure check on these cylinders.

Interpreting Compression Readings

1. If compression improves considerably, piston rings are at fault.

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 cause may be a head gasket leak between cylinders. Engine oil and/or coolant in cylinders could result from this condition.

Check the compression pressure limit chart when checking cylinder compression.

Example

If, after checking the compression pressures in all cylinders, it was found that the highest reading obtained was 1351 kPa (196 psi) and the lowest pressure reading was 1068 kPa (155 psi), the engine is within specification and the compression is considered satisfactory.