Engine Compression Test (1)

Engine Mechanical - 5.3L L84 or 6.2L L87 | Diagnostic Information and Procedures | Document ID: 5607933 Engine Compression Test Special Tools EN-22794 Spark Plug Port Adapter Equivalent regional tools: Special Tools A compression pressure test of the engine cylinders determines the condition of the piston rings, the valves and the head gaskets. With Scan Tool

	Step	With Scan Tool
	Note: Cylinder compression concerns may only be present when the engine and components are within a particular operating temperature range. If this is suspected to be the case, run the engine until it is near the temperature that the suspected issue is occuring. If engine can be started safely without causing damage, run the engine for at least couple of minutes to fully lubricate all engine components and systems. The battery must be at or near full charge. You may wish to connect a battery charger for the duration of the test to maintain an adequate battery level.
	1	Turn the engine OFF.
	2	Remove the spark plug wire(s) and spark plug(s) of the suspected cylinder(s). Spark Plug Wire Replacement - Left Side Gas Engine Ignition Spark Plug Replacement - Left Side Spark Plug Wire Replacement - Right Side Gas Engine Ignition Spark Plug Replacement - Right Side
	3	Connect the GDS2 scan tool 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 this test.
	4	Measure the engine compression, using the following procedure: 1. Thread the EN-22794 Spark Plug Port Adapter into the spark plug hole of the suspected cylinder(s). 2. Thread a compression pressure gauge into the EN-22794 Spark Plug Port Adapter. 3. Enable cranking via the GDS2. Record the compression pressure reading after engine cranking is complete. Note: When the compression measurement is normal, the compression builds up quickly and evenly to the specified compression on each cylinder.
	5	Replace the removed spark plug and repeat the compression test on two cylinders that are performing normally.

Step	With Scan Tool
	Note: If possible, choose cylinders to be tested in this step that are not adjacent to the suspect cylinder.
6	Compare the compression readings from the tested cylinders. 1. The lowest reading should not be less than 80 percent of the highest reading. 2. No cylinder reading should be less than 862 kPa (125 PSI). Note: If the compression pressure of the suspected cylinder(s) is near the 80 percent threshold, further testing should be performed to attempt to identify worn or damage piston rings, worn or damaged valves, and/or a worn or damaged cylinder head gasket.
7	Perform a cylinder leakage test. Cylinder Leakage Test Note: 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. Add approximately 15 ml (1 Tbs) 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.
8	Install all removed components.

Without Scan Tool

	Step	Without Scan Tool
	Note: Cylinder compression concerns may only be present when the engine and components are within a particular operating temperature range. If this is suspected to be the case, run the engine until it is near the temperature that the suspected issue is occuring. If engine can be started safely without causing damage, run the engine for at least couple of minutes to fully lubricate all engine components and systems. The battery must be at or near full charge. You may wish to connect a battery charger for the duration of the test to maintain an adequate battery level.
	1	Turn the engine OFF.
	2	Remove the spark plug wire(s) and spark plug(s) of the suspected cylinder(s). Spark Plug Wire Replacement - Left Side Gas Engine Ignition Spark Plug Replacement - Left Side Spark Plug Wire Replacement - Right Side Gas Engine Ignition Spark Plug Replacement - Right Side
	3	Measure the engine compression, using the following procedure: 1. Disable spark to all cylinders.

Step	Without Scan Tool
	2. Thread the EN-22794 Spark Plug Port Adapter into the spark plug hole of the suspected cylinder(s). 3. Thread a compression pressure gauge into the EN-22794 Spark Plug Port Adapter. 4. Engage the parking brake. Note: Fully depressing the accelerator pedal eliminates the delivery of fuel to the engine and holds the throttle body plate in a wide open throttle (WOT) position. 5. Fully depress, and hold, the brake pedal and the accelerator pedal. 6. While continuing to hold the brake and accelerator pedal, crank the engine with the ignition key or start button. Allow the engine to crank 4 times and turn off the ignition key or press the start button again to stop cranking the engine. 7. Record the compression pressure reading after engine cranking is complete.
4	Replace the removed spark plug and repeat the compression test on two cylinders that are performing normally. Note: If possible, choose cylinders to be tested in this step that are not adjacent to the suspect cylinder.
5	Compare the compression readings from the tested cylinders. 1. The lowest reading should not be less than 80 percent of the highest reading. 2. No cylinder reading should be less than 862 kPa (125 PSI). Note: If the compression pressure of the suspected cylinder(s) is near the 80 percent threshold, further testing should be performed to attempt to identify worn or damage piston rings, worn or damaged valves, and/or a worn or damaged cylinder head gasket.
6	Perform a cylinder leakage test. Cylinder Leakage Test Note: 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. Add approximately 15 ml (1 Tbs) 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.
7	Enable spark to all cylinders.
8	Release the parking brake.
9	Install all removed components.