Perform a compression test on all six cylinders of the 6.7L Cummins diesel engine to diagnose engine mechanical condition and identify issues with rings, valves, or head gasket.

Warnings

Tools required

Parts

• Glow plug sealing washers × 6 — Use OEM specification

Preparation

1. Park vehicle on level surface and apply parking brake
2. Allow engine to reach warm operating temperature (160-180°F), then shut off and allow to cool for 15-20 minutes
3. Open hood and disconnect both negative battery cables to prevent accidental starting
4. Remove engine cover by releasing retaining clips
5. Label and photograph all glow plug wire connections for correct reinstallation

Procedure

1. 1

   Disconnect glow plug electrical connectors
   Disconnect the electrical connector from each of the six glow plugs. These are located on the driver side of the cylinder head. Press the release tab and pull straight back to avoid damaging the connector. Mark or tag each connector position if not using original routing.
2. 2

   Remove glow plugs
   Using the appropriate glow plug socket, carefully remove all six glow plugs from the cylinder head. Turn counterclockwise with steady pressure. If a glow plug resists removal, apply penetrating oil and wait 10-15 minutes before attempting again. Do not force, as glow plugs can break off in the cylinder head. Inspect each glow plug tip for damage or carbon buildup.
   ⚠Apply only steady, even pressure when removing glow plugs. If excessive resistance is felt, stop and apply penetrating oil to avoid breaking the glow plug in the cylinder head.
3. 3

   Clean glow plug ports
   Use a wire brush or compressed air to clean any carbon deposits from the glow plug threads in the cylinder head. Ensure no debris falls into the cylinders. Wipe the area clean with shop towels.
4. 4

   Install compression tester adapter
   Thread the appropriate adapter from your diesel compression tester kit into the glow plug hole of cylinder #1 (front cylinder on driver side). Hand-tighten the adapter firmly to ensure a good seal, but do not overtighten. Attach the compression gauge hose to the adapter.
5. 5

   Disable fuel system
   Locate the fuel pump relay or fuse in the power distribution center and remove it to prevent fuel injection during cranking. This is critical for safety and accurate readings. On 2014 Ram 2500 Cummins, remove the fuel pump relay from the PDC located in the engine compartment.
   ⚠️Fuel system MUST be disabled before cranking engine with glow plugs removed to prevent fuel injection into open cylinders and potential fire hazard.
6. 6

   Perform compression test on cylinder #1
   Ensure compression tester gauge reads zero before starting. Have an assistant crank the engine using the key (or use a remote starter switch) for approximately 5-6 compression strokes or until the gauge needle stops climbing. Record the maximum PSI reading. Typical good compression for 6.7L Cummins is 400-500 PSI. Release pressure from the gauge using the valve on the tester.
7. 7

   Test remaining cylinders
   Remove the compression tester adapter from cylinder #1 and repeat the testing process for cylinders #2 through #6, moving from front to rear on the driver side. Record each cylinder's maximum compression reading. Ensure consistent cranking duration (same number of compression strokes) for each cylinder to ensure accurate comparison.
8. 8

   Analyze compression results
   Compare all six cylinder readings. All cylinders should read above 350 PSI minimum, with variation between highest and lowest not exceeding 10% of the highest reading. If one or more cylinders show low compression, perform a wet compression test by adding approximately 1 tablespoon of clean engine oil into the low cylinder through the glow plug hole and re-testing. If compression increases significantly, piston rings are likely worn. If compression remains low, suspect valve or head gasket issues.
9. 9

   Reinstall fuel system relay
   Replace the fuel pump relay or fuse that was removed in step 5 to restore normal fuel system operation.
10. 10

    Install new glow plug sealing washers
    Place a new sealing washer on each glow plug. Apply a small amount of anti-seize compound to the threads of each glow plug, being careful not to get any on the tip that extends into the cylinder.
11. 11

    Reinstall glow plugs
    Thread each glow plug by hand into its respective cylinder head port to avoid cross-threading. Once hand-tight, torque each glow plug to 15 Nm (11 lb-ft) using a torque wrench. Work systematically from cylinder #1 to #6.
    Torque spec

    Sensor Bolts15 Nm (11 lb-ft)
12. 12

    Reconnect glow plug electrical connectors
    Reconnect the electrical connector to each glow plug, ensuring each connector clicks into place securely. Verify correct connector routing matches your reference photos.
13. 13

    Reconnect battery and reinstall engine cover
    Reconnect both negative battery cables. Reinstall the engine cover, ensuring all retaining clips are properly seated.

Reassembly

1. Double-check all glow plug electrical connections are secure before starting engine
2. Verify fuel pump relay/fuse has been reinstalled
3. Ensure no tools or parts are left in engine compartment

Verification

• Start engine and verify it runs smoothly without misfires or unusual noises
• Check for exhaust leaks around glow plug ports while engine is running
• Verify glow plug indicator light functions normally on instrument cluster during next cold start
• Review compression test data: all cylinders should be above 350 PSI with less than 10% variation between highest and lowest readings

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