Compression Check: Testing and Inspection

Component Tests


Compression Test


General Remarks

NOTE:
Removing fuses and disconnecting electrical components may cause the Engine Control Module (ECM) to log Diagnostic Trouble Codes (DTCs). After the measurements have been carried out, DTCs should be cleared from memory by connecting to the Manufacturer Approved Diagnostic System.

NOTE:
Only check the compression pressure with the valves set to the prescribed clearance (if this can be adjusted).
The compression pressure should be checked with the engine at operating temperature.

Check the Compression Pressure

WARNING: Move gear selector lever to 'P' position. Failure to follow this instruction may result in personal injury.
1 Remove the fuel pump relay.
2 Start the engine - the engine will start, run for a few seconds then stop.
3 Remove the spark plugs.
4 Install the compression tester.
5 Install an auxiliary starter switch in the starting circuit. With the ignition switch OFF, using the auxiliary 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.
7 Install the removed components in reverse order, observing the specified tightening torques.
8 Clear all DTCs from the ECM.

Interpretation of the Results

NOTE:
Due to the possibility of loose carbon that has become trapped between the valve face and seat effecting the pressure readings, when carrying out a compression test and cylinders are found to have low pressures, install the spark plugs, road test the vehicle and re-test the suspect cylinders. If the correct pressures are restored, no further action is required.
The indicated compression pressures are considered within specification if the lowest reading cylinder is within 75% of the highest reading.
If the cylinder pressures are found to be low, carry out a leakdown test to determine the location of the fault (if any leakback can be heard through the engine breather system suspect the piston rings, if any leakback can be heard through the inlet system suspect the inlet valve or seat, if any leakback can be heard through the exhaust manifold suspect the exhaust valve or seat. If the measurements for two cylinders next to each other are both too low then it is very likely that the cylinder head gasket between them is burnt through. This can also be recognized by traces of engine oil in the coolant and/or coolant in the engine oil).