EN-51749 Compression Gauge Tester Kit

For equivalent regional tools, refer to Special Tools.

A compression pressure test of the engine cylinders determines the condition of the piston rings, the valves and the head gaskets.

Note: The battery must be at or near full charge.

1. Connect a battery charger to the battery for the duration of the test to maintain an adequate battery level.
2. Engine OFF.
3. Remove the spark plugs. Refer to Gas Engine Ignition Spark Plug Replacement.
4. Interrupt the fuel supply.

1. 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.

Note: Certain Diagnostic Troubles Codes will disable the compression test and other output functions in GDS2. If there are any DTCs set, record the failure records and or freeze frame data before clearing the faults.

2. Clear all DTCs.
3. If cranking control is not available through GDS2, use a remote start switch in place of the Starter Relay to control cranking. When using a remote start switch to crank the engine keep the GDS2 compression test function enabled.
4. For vehicles with Start/Stop System RPO KL9 Only: If using a remote start switch in place of the Starter Relay, it will also be necessary to remove the Starter Pinion Solenoid Actuator Relay KR27C. Use an appropriate sized fused jumper wire in place of the KR27C relay in order to actuate the starter pinion, refer to Starting and Charging Schematics. Proceed with cranking the engine using the Remote Start Switch.



5. Install the EN-51749 gauge (1, 2) into the spark plug hole for the cylinder that is being checked.

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.

6. 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.
7. Repeat the compression test for each cylinder.
8. Compare the compression readings from all of the cylinders. Maximum pressure differential 100 kPa (14.5 psi).
9. 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.
10. Disconnect GDS2 diagnostic tool, remote start switch, and any fused jumper leads.

1. Restore the fuel supply.
2. If removed, install the Starter Pinion Solenoid Actuator Relay.
3. Install the spark plugs. Refer to Gas Engine Ignition Spark Plug Replacement.