Leak-Down Test

This procedure performs a cylinder leak-down test on the 5.9L Cummins diesel engine to diagnose internal engine condition by measuring compressed air loss through valves, rings, or gaskets.

Warnings

Tools required

Parts

• Glow plug copper washers/seals × 6 — Set of 6, use OEM specification

Preparation

1. Ensure engine is completely cold (at ambient temperature)
2. Park vehicle on level surface and engage parking brake
3. Disconnect both negative and positive battery cables to prevent accidental starter engagement
4. Remove engine cover if equipped
5. Remove air intake system from turbocharger inlet to air box
6. Clearly label all glow plug wire connections before removal

Procedure

1. 1

   Remove air intake system
   Loosen clamps on air intake tubing from air box to turbocharger inlet. Remove air box assembly by releasing clips and disconnecting any sensors. Remove intake tubing to provide access to valve cover area.
   Torque spec

   Air Box Clips5 Nm (4 lb-ft)
2. 2

   Remove valve cover
   Disconnect fuel injector wiring harness connector and any sensor connectors at the valve cover as needed for access. Remove valve cover bolts in reverse sequence (outside to inside pattern). Carefully lift valve cover away from cylinder head. Inspect gasket condition and clean mating surfaces if gasket is disturbed.
   Torque spec

   Connector Bolts7 Nm (5 lb-ft)
3. 3

   Remove all glow plugs
   Using the appropriate tools, carefully remove all six fuel injectors (or injector hold-down assemblies) to access each cylinder. Note which cylinder each injector came from and inspect the injector nozzle/bore condition. Clean injector bores with compressed air. Fuel injectors can seize in the cylinder head bore. Use a proper injector puller and penetrating oil if necessary; do not force removal.
   ⚠Glow plugs can break off in cylinder head if seized. Apply heat and penetrating oil if necessary before forcing removal.
4. 4

   Rotate engine to cylinder 1 TDC compression
   Using breaker bar and harmonic balancer socket on crankshaft bolt, rotate engine clockwise (viewed from front) until timing marks on harmonic balancer align with TDC mark on timing cover. Verify cylinder 1 is on compression stroke by confirming both intake and exhaust rocker arms are loose (valves closed) with the valve cover off. If rocker arms are tight, rotate engine one additional full revolution to TDC compression stroke.
5. 5

   Install leak-down tester adapter to cylinder 1
   Thread the leak-down tester injector-bore adapter fitting into cylinder 1 injector bore. Ensure adapter is fully seated and secured per adapter instructions. Connect leak-down tester gauge assembly to adapter. Set regulator on leak-down tester to 100 PSI per manufacturer instructions.
6. 6

   Perform cylinder 1 leak-down test
   Slowly apply compressed air through leak-down tester while monitoring both gauges. Record percentage of leakage shown on gauge (typically displayed as difference between input and output pressure). Acceptable leakage is typically 10% or less, marginal is 10-20%, and excessive is over 20%. Listen at oil filler, exhaust tailpipe, and intake for sources of air leakage. Listen at radiator cap for head gasket leakage. Document results.
   ⚠Engine may attempt to rotate if piston is not exactly at TDC. Keep hands clear of fan and belts during air application.
7. 7

   Test remaining cylinders in sequence
   Release air pressure from tester and remove adapter from cylinder 1. Rotate crankshaft 120 degrees clockwise to bring next cylinder in firing order (1-5-3-6-2-4) to TDC compression stroke. For 120 degree rotation, make reference marks on balancer every 120 degrees. Install adapter, verify TDC compression stroke position, and repeat leak-down test. Continue for all six cylinders, documenting results for each cylinder including leakage percentage and source of leakage if detected.
8. 8

   Analyze leak-down test results
   Compare leak-down percentages across all cylinders. Consistent readings indicate uniform engine condition. High leakage at exhaust indicates exhaust valve issues. High leakage at intake indicates intake valve issues. Leakage at oil filler indicates piston ring wear. Leakage at adjacent cylinder or coolant system indicates head gasket failure. Document findings and recommendations based on results.
9. 9

   Reinstall glow plugs
   Install new injector seals/O-rings and copper sealing washers on each injector. Carefully install each fuel injector into its respective bore by hand. Torque the injector hold-down bolts and fuel line connections to the manufacturer's specification. Do not overtighten as this can damage the injector or seat.
10. 10

    Reinstall valve cover
    Clean valve cover and cylinder head mating surfaces thoroughly. Install new valve cover gasket if original was damaged during removal. Position valve cover onto cylinder head and install cover screws hand-tight. Torque cover screws to the Cummins-specified valve cover bolt torque in a crisscross pattern from center outward. Reconnect all electrical connectors to the fuel injector harness and any sensors.
    Torque spec

    Connector Bolts7 Nm (5 lb-ft)
11. 11

    Reinstall air intake system
    Reinstall intake tubing from turbocharger to air box. Secure all clamps and reconnect any sensors that were disconnected. Install air box assembly and secure clips. Reinstall engine cover if equipped.
    Torque spec

    Air Box Clips5 Nm (4 lb-ft)

Reassembly

1. Ensure all electrical connectors are properly seated and locked
2. Verify all intake system clamps are tight to prevent boost leaks
3. Double-check that all glow plugs are installed and torqued correctly
4. Reconnect battery cables (positive first, then negative)

Verification

• Turn ignition key to ON position (do not start) and verify glow plug indicator light functions normally
• Start engine and listen for any unusual noises or misfires
• Check for intake air leaks around air box and intake tubing connections
• Verify no oil leaks around valve cover gasket
• Review leak-down test results: readings under 10% indicate good engine condition, 10-20% indicates wear, over 20% indicates significant internal engine problems requiring further diagnosis

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