A leak-down test evaluates cylinder sealing by pressurizing each cylinder with compressed air and measuring pressure loss, helping diagnose piston ring wear, valve seating issues, or head gasket leaks on the 6.7L Cummins diesel engine.

Warnings

Tools required

Parts

• Glow plug sealing washers × 6 — 68088889AA or equivalent

Preparation

1. Ensure engine is completely cold (preferably overnight)
2. Park vehicle on level ground and engage parking brake
3. Disconnect both battery negative terminals to prevent starter engagement
4. Remove engine cover and air intake system for access to glow plugs
5. Label all glow plug wire connections for correct reinstallation
6. Verify compressed air supply delivers stable 90-100 PSI
7. Calibrate leak-down tester according to manufacturer instructions

Procedure

1. 1

   Remove air intake components
   Release air box clips and remove the air filter housing assembly. Disconnect intake piping from the turbocharger inlet. Remove any brackets or sensors attached to the intake system that obstruct access to the valve cover area. Store all fasteners and clips in organized containers.
   Torque spec

   Air Box Clips5 Nm (4 lb-ft)

   Bracket Bolts27 Nm (20 lb-ft)

   Sensor Bolts15 Nm (11 lb-ft)
2. 2

   Access and remove glow plugs
   Disconnect the glow plug electrical connectors from all six cylinders. Using a 10mm deep-well socket or dedicated glow plug socket, carefully remove all six glow plugs. Turn slowly and steadily to avoid breaking the fragile glow plug tips. Inspect each glow plug for carbon buildup or damage. Keep plugs organized by cylinder number.
   ⚠If glow plugs show resistance when removing, apply penetrating oil and wait 15 minutes before continuing - forcing can break plugs in cylinder head
3. 3

   Rotate engine to cylinder 1 TDC compression stroke
   Install the harmonic balancer holding tool or access the flywheel through the bell housing inspection cover. Manually rotate the engine clockwise using a socket on the crankshaft bolt until cylinder #1 is at top dead center on the compression stroke. Verify both intake and exhaust valves are closed (rocker arms loose). The timing mark on the balancer should align with the zero mark on the timing cover.
   ℹ️Cylinder #1 is at the front of the engine on the driver side
4. 4

   Install leak-down tester on cylinder 1
   Thread the leak-down tester adapter fitting into cylinder #1 glow plug hole hand-tight. Connect the leak-down tester gauge assembly to the adapter. Lock the crankshaft in position using the holding tool to prevent rotation. Ensure the air supply regulator is set to 90-100 PSI as specified by your leak-down tester.
   ⚠️Crankshaft MUST be locked before applying air pressure to prevent engine rotation
5. 5

   Perform leak-down test on cylinder 1
   Apply compressed air gradually and observe both gauges on the leak-down tester. Record the percentage of leakage shown on the gauge. Listen at the oil fill cap (crankcase leakage past rings), tailpipe (exhaust valve leakage), intake manifold (intake valve leakage), and coolant overflow (head gasket leakage). Use a mechanics stethoscope for better leak location. Note the leak percentage and source of any air loss.
   ℹ️Typical acceptable leak-down is 5-10%; 10-15% is marginal; above 15% indicates wear or damage requiring attention
6. 6

   Test cylinder 2
   Release air pressure and remove the leak-down tester from cylinder 1. Rotate the crankshaft 120 degrees clockwise (the 6.7L Cummins has a 120-degree firing interval). Verify cylinder #2 is at TDC compression stroke with both valves closed. Install leak-down tester, lock crankshaft, apply air pressure, and record results including leak percentage and source.
7. 7

   Test cylinder 3
   Release air pressure and remove tester. Rotate crankshaft another 120 degrees clockwise to position cylinder #3 at TDC compression stroke. Verify both valves are closed. Install tester, lock crankshaft, pressurize, and record leak-down percentage and any detectable leak paths.
8. 8

   Test cylinder 4
   Release air pressure and remove tester. Rotate crankshaft 120 degrees clockwise to cylinder #4 TDC compression stroke. Confirm valve closure, install tester, secure crankshaft, apply air pressure, and document leak-down results.
9. 9

   Test cylinder 5
   Release air pressure and remove tester. Rotate crankshaft 120 degrees clockwise to cylinder #5 TDC compression stroke. Verify both valves closed, install tester, lock crankshaft, pressurize, and record leak-down percentage and leak sources.
10. 10

    Test cylinder 6
    Release air pressure and remove tester. Rotate crankshaft final 120 degrees clockwise to cylinder #6 TDC compression stroke. Confirm valves are closed, install tester, secure crankshaft, apply air pressure, and complete final cylinder leak-down documentation.
11. 11

    Analyze results and reinstall glow plugs
    Review all six cylinder leak-down percentages. Compare cylinders to identify outliers. Variation greater than 5% between cylinders may indicate specific cylinder problems. Apply anti-seize compound sparingly to glow plug threads. Install new sealing washers on each glow plug. Thread glow plugs by hand into each cylinder carefully to avoid cross-threading, then tighten to 11 lb-ft (15 Nm) using a torque wrench.
    ⚠Do not over-torque glow plugs - the aluminum cylinder head threads are easily damaged
    Torque spec

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

    Reconnect glow plug electrical system
    Reconnect the glow plug electrical connectors to each cylinder using the labels created during disassembly. Ensure each connector clicks securely into place and wiring is routed away from hot exhaust components.
13. 13

    Reinstall air intake system
    Reinstall all intake components in reverse order of removal. Connect intake piping to turbocharger inlet. Secure all brackets and sensors removed during disassembly. Install air filter housing and secure with air box clips. Verify all connections are tight and no components are left disconnected.
    Torque spec

    Air Box Clips5 Nm (4 lb-ft)

    Bracket Bolts27 Nm (20 lb-ft)

    Sensor Bolts15 Nm (11 lb-ft)

Reassembly

1. Reconnect both battery negative terminals
2. Install engine cover
3. Clear any diagnostic codes that may have set from disconnecting sensors
4. Document all leak-down test results with cylinder number, percentage, and leak source for maintenance records

Verification

• Start engine and verify smooth operation with no misfires
• Check for any air leaks around glow plug installations
• Verify glow plug indicator lamp functions normally on key-on cycle
• Review leak-down results: percentages should be consistent across cylinders (within 5%), with healthy cylinders showing 5-10% leakage or less
• If any cylinder exceeds 15% leakage or shows significant variation from others, further diagnosis is required to determine repair needs

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