Compression Check: Testing and Inspection

Compression Test

1. Make sure oil in crankcase is of the correct viscosity and at the proper level and that the battery is properly charged. Operate the vehicle until the engine is at normal operating temperature. Turn the ignition switch to the OFF position, then remove the spark plugs.
2. Place the transmission in P (Park) (A/1) or N (Neutral) (M/T) and apply the parking brake.
3. Block throttle plate in the wide-open position.
4. Install a compression gauge in the No.1 cylinder spark plug hole.
5. Install a remote starter switch in the starting circuit. With the ignition switch in the OFF position, and using the remote starter switch, 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.
6. Repeat the test on each cylinder, cranking the engine approximately the same number of compression strokes.

Compression Test - Interpreting Compression Readings

1. If compression improves considerably, piston rings require service.
2. If compression does not improve, valves are sticking or seating improperly.
3. If two adjacent cylinders indicate low compression pressures and squirting oil on piston does not increase compression, the head gasket may be leaking between cylinders. Engine oil and/or coolant in the cylinders could result from this condition. Use the Compression Pressure Limit Chart when comparing cylinder compression readings to determine if the lowest reading is at least 75 percent of the highest reading.

Compression Test - Results
The indicated compression pressures are considered within specification if the lowest reading cylinder is at least 75 percent of the highest reading. The following Compression Pressure Limit Chart shows acceptable pressure ranges. Use the highest compression reading obtained from the compression test and verify that the lowest compression reading is within the range shown in the chart. See the Compression Pressure Limit Chart.

Compression Pressure Limit Chart

Compression Pressure Limit Chart:

If one or more cylinders reads low, squirt approximately one tablespoon of clean 10W40 engine oil on top of the piston(s) in the low-reading cylinder(s). Repeat the compression pressure check on these cylinders.