Envoy 4WD V8-5.3L VIN M (2005)
Compression Check: Testing and Inspection
Displacement On Demand (DoD) System Compression Test
Displacement on Demand (DoD) System Compression Test
The Displacement-on-Demand (DoD) system compression test is a functional test of the valve lifter oil manifold (VLOM) and valve lifters. Using a
compression gauge, the technician can monitor the specific cylinders ability to go into and out of DoD as commanded by the scan tool. The actual
pressure values on the compression gage are not as critical as is observing the on-off switching action of the solenoid and valve lifters.
1. Check the engine oil level. Fill as required.
2. Perform the Diagnostic System Check. Refer to Diagnostic System Check - Vehicle. See: Testing and Inspection/Initial Inspection and Diagnostic
Overview/Diagnostic System Check - Vehicle
3. Disable the ignition system for the cylinder to be tested, by disconnecting the electrical wire harness to the ignition coil.
4. Disable the fuel injection system for the cylinder to be tested, by disconnecting the electrical wire harness to the fuel injector.
5. Remove the spark plug and wire for the cylinder to be tested.
6. Install a 0 - 1378 kPa (0 - 200 psi) compression gage.
7. Start the engine.
8. Using the scan tool output controls, command the solenoid ON, for the cylinder to be deactivated.
9. Depress the Schrader valve on the compression gage in order to release the pressure and zero the gage. With the engine running and the cylinder in
DoD mode, the compression reading should be less than 172 kPa (25 psi).
10. Using the scan tool, de-energize the solenoid while observing the reading on the compression gage. With the engine running and the cylinder NOT
in DoD mode, the compression gage reading should increase quickly to greater than 345 kPa (50 psi).
11. Important: Only one cylinder can be tested at a time. When testing has been completed on a specific cylinder, the wire harness electrical
connectors, spark plug, and spark plug wire must be installed prior to testing of each additional cylinder.
Test each of the four DoD cylinders one at a time. If the readings are not within specifications, remove and test the VLOM as required.
12. If the compression test has been performed and all DoD cylinders are operating within specifications, the DoD system is performing as designed
and no repairs are required.
13. Clear the DTCs with a scan tool.
