Section 03-00: Engine—Service
1993 Mustang Workshop Manual
DIAGNOSIS AND TESTING

Compression Test

Tools Required:

 

Compression Gauge Check

  1. Ensure 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.
  1. Set throttle plate in wide-open position.
  1. Install a compression gauge such as Rotunda Compression Tester 059-00009 or equivalent in No. 1 cylinder.
  1. Install an auxiliary starter switch in starting circuit. With ignition switch in the OFF position, and using auxiliary starter switch, crank engine at least five compression strokes and record highest reading. Note the approximate number of compression strokes required to obtain the highest reading.
  1. Repeat test on each cylinder cranking the engine approximately 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 Compression Pressure Limit Chart.

Compression Pressure Limit Chart




If one or more cylinders read low, squirt approximately one tablespoon of XO-20W50-QR (ESE-M2C179-A) 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.
  1. If compression does not improve, valves are sticking or seating poorly.
  1. 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 condition.

    It is recommended the Compression Pressure Limit Chart be used when checking cylinder compression 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.