Compression Test

This procedure tests the compression of each cylinder in the 5.9L Cummins diesel engine to diagnose engine condition, worn rings, valve issues, or head gasket problems.

Warnings

Tools required

Parts

• Glow plug copper sealing washers (if reusing glow plugs) × 6 — Use OEM specification

Preparation

1. Warm engine to normal operating temperature, then shut off and allow to cool for 15-20 minutes
2. Ensure battery is fully charged or connect a battery charger to maintain voltage during extended cranking
3. Park vehicle on level surface and engage parking brake
4. Open hood and allow engine bay to ventilate
5. Gather all tools and ensure compression tester is calibrated and functioning

Procedure

1. 1

   Disable fuel delivery system
   Locate the fuel shut-off solenoid on the injection pump (P7100 pump: driver's side; VP44: on the pump body). Disconnect the electrical connector from the fuel shut-off solenoid to prevent fuel delivery during cranking. This ensures the engine will not attempt to start during the test.
2. 2

   Remove air intake system
   Remove the air filter housing and intake tubing to access the top of the engine. Release the air box clips and disconnect any sensors or clamps. Set components aside in a clean area.
   Torque spec

   Air Box Clips5 Nm (4 lb-ft)
3. 3

   Locate and access glow plugs
   The fuel injectors are located along the top of the cylinder head. To access a cylinder for compression testing, the injector for that cylinder must be removed. Disconnect and remove the fuel injector lines and injector hold-downs as required. Label or photograph line positions if needed for reassembly.
4. 4

   Remove glow plugs
   Remove the fuel injector from the cylinder to be tested. Loosen the injector hold-down and high-pressure line, then carefully extract the injector. Take care not to damage the injector or its sealing washer. Note the condition of each injector and its sealing washer.
5. 5

   Install compression tester adapter
   Install the appropriate diesel compression tester adapter into the injector bore of cylinder #1 (front of engine). Use the correct injector-bore adapter for the 5.9L Cummins and hand-tighten firmly to ensure a proper seal.
6. 6

   Test cylinder #1 compression
   Connect the compression tester to the adapter. Have an assistant crank the engine for approximately 5-6 compression strokes (about 5 seconds of cranking). Watch the gauge and note when the reading stabilizes. Record the maximum pressure reading. Release pressure from the tester using the valve on the gauge.
7. 7

   Test remaining cylinders
   Remove the compression tester adapter and reinstall the cylinder #1 injector, then remove the cylinder #2 injector and install the adapter. Repeat the testing process for all six cylinders in order from front to back. Record each reading. Maintain consistent cranking speed and duration for each cylinder to ensure comparable results.
8. 8

   Analyze compression results
   Compare all cylinder readings. Consult Cummins service data for the specific engine; readings on a healthy 5.9L are typically in the range of 350-500 PSI with no more than approximately 10-15% variation between highest and lowest cylinders. Low compression in one cylinder indicates issues with that cylinder (worn rings, valve problems). Low compression in adjacent cylinders may indicate head gasket failure. Uniformly low compression across all cylinders suggests worn piston rings or overall engine wear.
9. 9

   Clean and reinstall glow plugs
   Clean the injector bores and sealing surfaces. Install new injector sealing washers on each injector. Reinstall each injector into its respective bore, ensuring proper alignment. Reinstall the hold-down clamps and torque them to Cummins specification, then reconnect and torque the high-pressure fuel lines. Do not overtighten. as glow plugs are fragile.
10. 10

    Reconnect electrical connections
    Reconnect and torque all fuel injector high-pressure lines and any injector electrical connections. Reconnect the fuel shut-off solenoid electrical connector on the injection pump.
11. 11

    Reinstall air intake system
    Reinstall the air intake tubing and air filter housing. Secure all clips, clamps, and sensor connections. Verify all connections are tight and properly seated.
    Torque spec

    Air Box Clips5 Nm (4 lb-ft)

Reassembly

1. Double-check that all glow plug electrical connectors are firmly seated
2. Verify fuel shut-off solenoid is reconnected and fuel delivery is enabled
3. Ensure no tools or rags are left in the engine bay

Verification

• Start the engine and verify it runs smoothly without misfires or unusual noises
• Check for any exhaust leaks around the glow plug locations
• Verify all compression readings were within normal range (350-400 PSI) with less than 10% variation between cylinders
• If compression was low or uneven, diagnose root cause: single cylinder low indicates valve or ring issue in that cylinder, adjacent cylinders low indicates possible head gasket failure, all cylinders uniformly low indicates general engine wear

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