Cylinder Deactivation (Active Fuel Management) System Compression Test

Cylinder Deactivation (Active Fuel Management) System Compression Test

Important: Active Fuel Management System Compression Test is applicable for RPO LC9/LY5/LMG/L76/L77.


The Active Fuel Management System Compression Test is a functional test of the valve lifter oil manifold and valve lifters. Using a compression gauge, the technician can monitor the specific cylinders ability to go into and out of active fuel management as commanded by the scan tool. The actual pressure values on the compression gauge are not as critical as is observing the on-off switching action of the solenoid and valve lifters.

1. Measure the engine oil level. Fill, as required.
2. Perform the Diagnostic System Check - Vehicle. Refer to Diagnostic System Check - Vehicle 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 0-1 378 kPa (0-200 psi) compression gauge.
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 gauge, in order to release the pressure and zero the gauge. With the engine running and the cylinder in active fuel management 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 gauge. With the engine running and the cylinder NOT in active fuel management mode, the compression gauge reading should increase quickly to greater than 345 kPa (50 psi).

Important: Only 1 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.


11. Test each of the 4 active fuel management cylinders, 1 at a time. If the readings are not within specifications, remove and test the valve lifter oil manifold, as required. Refer to Valve Lifter Oil Manifold Replacement Valve Lifter Oil Manifold Replacement and Cylinder Deactivation (Active Fuel Management) Valve Lifter Oil Manifold Diagnosis and Testing Cylinder Deactivation (Active Fuel Management) Valve Lifter Oil Manifold Diagnosis and Testing.
12. If the compression test has been performed and all active fuel management cylinders are operating within specifications, the active fuel management system is performing as designed and no repairs are required.
13. Clear the DTCs with a scan tool.