Compression Check: Testing and Inspection

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

Compression Pressure Limit Chart:

Compression Gauge Check

1. Ensure oil in crankcase is at proper level and battery is properly charged. Operate vehicle until engine is at normal operating temperature. Turn off ignition switch, then remove all spark plugs.

2. Set carburetor throttle plates and choke plate in wide-open position.

3. Install a compression gauge such as Rotunda Compression Tester O59-OOOO9 or an equivalent in No.1 cylinder.

4. Install an auxiliary starter switch in starting circuit. With ignition switch off and using auxiliary starter switch crank engine at least five compression strokes and record highest reading.

5. Repeat test on each cylinder as required to obtain highest reading on No.1 cylinder.

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 Compression Pressure Limit Chart below. Specifications

If one or more cylinders read low, squirt approximately one tablespoon of heavy SAE 50 Weight or equivalent engine oil on top of the pistons in the low reading cylinders. Repeat compression pressure check on these cylinders.

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

It is recommended the Compression Pressure Limit Chart be used when checking cylinder compression pressures. The chart has been calculated so that the lowest reading number is 75 percent of the highest reading.

Example

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