Leak-Down Test

A leak-down test measures cylinder sealing by pressurizing each cylinder with compressed air and measuring pressure loss to diagnose piston ring wear, valve sealing issues, or head gasket failures.

Warnings

Tools required

Parts

• Glow plug copper sealing washers × 6 — Mopar 68005143AA or equivalent

Preparation

1. Ensure engine is completely cold for accurate testing
2. Park vehicle on level surface, apply parking brake, and place transmission in Park or Neutral
3. Disconnect both negative and positive battery cables to prevent accidental starter engagement
4. Remove engine cover if equipped
5. Locate all six glow plugs on driver side of cylinder head
6. Clean area around glow plugs thoroughly with compressed air to prevent debris from entering cylinders
7. Prepare a chart to record leak-down percentages and observations for cylinders 1-6

Procedure

1. 1

   Remove glow plugs from all cylinders
   Using the correct deep glow plug socket (typically 12mm hex for the 5.9L Cummins), carefully remove all six glow plugs. Work slowly to avoid breaking the fragile tips. Cummins 5.9L glow plugs typically seal without a separate copper washer; verify whether a sealing washer is present before removal and, if so, replace with the correct OEM part. Inspect glow plug tips for damage or excessive carbon buildup. Keep glow plugs organized by cylinder number for reinstallation.
2. 2

   Rotate engine to TDC for cylinder 1
   Using the correct crankshaft/barring socket for the 5.9L Cummins (verify size against service manual), rotate the engine clockwise (from front of engine) until cylinder 1 reaches top dead center on the compression stroke. You can verify TDC compression stroke by feeling air pressure escaping from cylinder 1 glow plug hole as you slowly approach TDC. The crankshaft damper TDC mark should align with the timing pointer.
3. 3

   Install leak-down tester in cylinder 1
   Thread the leak-down tester adapter into cylinder 1 glow plug hole by hand, ensuring proper thread engagement. Connect the leak-down tester to the adapter and to your compressed air source. Set your shop air regulator to 80-100 PSI as specified by your leak-down tester manufacturer's instructions.
4. 4

   Perform leak-down test on cylinder 1
   Apply compressed air pressure to cylinder 1 and observe both gauges on the leak-down tester. Read the percentage of leak-down from the gauge. Listen at the intake manifold, exhaust pipe, and oil fill cap for escaping air, and watch the coolant surface (reservoir/radiator with cap removed) for bubbles to diagnose leak location. Air at intake indicates intake valve leakage, exhaust indicates exhaust valve leakage, oil fill indicates piston ring leakage, and coolant bubbling indicates head gasket failure. Record all findings for cylinder 1.
5. 5

   Test remaining cylinders in sequence
   Remove the leak-down tester from cylinder 1. Rotate the crankshaft clockwise 120 degrees (one-third turn) to bring the next cylinder in the firing order to TDC on its compression stroke. For this inline 6-cylinder engine, the firing order is 1-5-3-6-2-4 (verify against service manual), so test in that sequence: cylinder 1, then 5, then 3, then 6, then 2, then 4. Repeat the leak-down test procedure for each cylinder, rotating 120 degrees between each test. Record results and observations for all six cylinders.
6. 6

   Analyze test results
   Compare leak-down percentages across all cylinders. Consistent readings of 5-10% indicate good engine health. Readings of 10-20% suggest wear but may be acceptable depending on mileage. Readings above 20% indicate significant problems. Note which cylinders show excessive leakage and where the air is escaping (intake, exhaust, crankcase, or cooling system) to determine the nature of the problem.
7. 7

   Reinstall glow plugs with new washers
   Install new copper sealing washers on each glow plug. Thread each glow plug into its respective cylinder by hand to avoid cross-threading. Torque glow plugs to specification using a torque wrench. Since no torque spec is provided in the database, torque glow plugs to the Cummins service manual specification (verify exact value with service manual before applying).
8. 8

   Reconnect electrical system
   Reconnect positive battery cable first, then negative battery cable. Ensure all connections are clean and tight. Reinstall engine cover if equipped.

Reassembly

1. Double-check that all six glow plugs are properly installed and torqued
2. Verify no tools or equipment remain in engine bay
3. Clear any diagnostic trouble codes that may have been set during testing

Verification

• Turn ignition to ON position and verify glow plug indicator light functions normally
• Start engine and listen for smooth operation with no misfires or unusual noises
• Check for exhaust smoke indicating possible compression or sealing issues
• Review recorded leak-down test results: normal engines should show 5-10% leakage with minimal variation between cylinders
• If any cylinder showed excessive leak-down, plan appropriate repairs based on leak source identified during testing

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