Measurement Procedure
WARNING: This page is about a different variant/trim than selected.
- Connect the GDS2 to the vehicle and select "Compression Test". Enabling the Compression Test will disable the fuel pump, fuel injectors, and spark. The throttle body is opened to a Wide Open Throttle (WOT) position. The GDS2 will also automatically control the cranking procedure for many applications. If cranking control is not available use a remote start switch in place of the Starter Relay to control cranking.
Install the EN-51749 gauge (1, 2) into the spark plug hole for the cylinder that is being checked.
- Use a remote start switch to crank the engine. If cranking function exists via GDS2, this can be used as an alternative method. Record the reading after the cranking cycle completes.
- Repeat the compression test for each cylinder.
- Compare the compression readings from all of the cylinders. Maximum pressure differential 100 kPa (14.5 psi).
- The following are indicators of potential problems:
- When low compression is caused by the piston rings, compression is low on the first stroke and tends to build up on the following strokes, but does not reach normal. Compression improves considerably with the addition of oil to the cylinder. Use approximately 3 squirts of oil from a plunger-type oiler into the cylinder and replace the gauge and measure again.
- When low compression is caused by the valves, the measurement is low on the first stroke and does not build up even with the addition of oil.
- Leaking head gaskets will provide the same results as worn valves but engine coolant may be identified in the crankcase. In addition, a leaking head gasket will give low readings on paired cylinders.
- Disconnect GDS2 diagnostic tool or remote start switch.
NOTE:
Engine should be operated at least 300 revolutions per minute (RPM).
NOTE:
When the compression measurement is normal, the compression builds up quickly and evenly to the specified compression on each cylinder.