Leak-Down Test

A leak-down test measures cylinder sealing efficiency by pressurizing each cylinder with compressed air and measuring the percentage of air loss to diagnose piston ring, valve, and head gasket issues.

Warnings

Tools required

Parts

• Spark plug set (8 plugs) × 1 — AC Delco 41-162 or equivalent

Preparation

1. Ensure engine is completely cold (ambient temperature) before beginning work
2. Park vehicle on level surface, engage parking brake, and place transmission in Park
3. Disconnect negative battery cable to prevent accidental starter engagement
4. Remove engine cover by pulling upward on cover (uses integral grommets)
5. Label all ignition coils with cylinder numbers (1-8) before removal to ensure correct reinstallation
6. Have leak-down tester calibrated and ready with compressed air source available
7. Prepare a recording sheet with columns for each cylinder number and corresponding leak-down percentage

Procedure

1. 1

   Remove ignition coils and spark plugs
   Disconnect the electrical connector from each ignition coil by pressing the release tab. Remove the 10mm bolt securing each coil and carefully pull coils straight up from spark plug wells. Using a spark plug socket, remove all eight spark plugs and inspect for fouling, oil, or coolant contamination which may indicate the source of leakage. Keep spark plugs in order if reusing for comparison purposes.
2. 2

   Rotate engine to TDC compression stroke for cylinder #1
   Using a 24mm socket on the crankshaft bolt, rotate the engine clockwise (from front of vehicle) while observing cylinder #1 spark plug hole. Place your thumb over the hole to feel compression building. Continue rotating until the piston is at the highest point and both intake and exhaust valves are closed (you should feel maximum resistance with your thumb). This is TDC compression stroke. The timing mark on the harmonic balancer should align with the timing tab on the front cover.
3. 3

   Install leak-down tester adapter and pressurize cylinder #1
   Thread the leak-down tester adapter into cylinder #1 spark plug hole by hand, ensuring it seats properly without cross-threading. Connect the leak-down tester to the adapter and to your compressed air source. Set the regulator to supply exactly 100 PSI to the cylinder through the tester. Observe the gauge reading which shows the percentage of air leaking from the cylinder. Record the leak-down percentage for cylinder #1. Normal readings are 5-10%, marginal is 10-20%, and above 20% indicates a problem requiring further diagnosis.
   ⚠If the piston is not precisely at TDC, air pressure may force the engine to rotate. If rotation occurs, remove pressure, re-establish TDC, and try again.
4. 4

   Diagnose leak source for cylinder #1
   While maintaining air pressure in the cylinder, listen and feel for air escaping to determine leak source. Air hissing from the throttle body indicates intake valve leakage. Air from the exhaust tailpipe indicates exhaust valve leakage. Air bubbling in the coolant expansion tank indicates head gasket or crack leakage. Air from the oil fill cap or crankcase vent indicates piston ring leakage. Document your findings alongside the leak-down percentage. Release air pressure and remove tester adapter.
5. 5

   Test cylinder #2
   Rotate the crankshaft clockwise approximately 90 degrees to bring cylinder #2 to TDC compression stroke (firing order is 1-8-7-2-6-5-4-3, so cylinders reach TDC in that sequence). Verify TDC by feeling for compression at cylinder #2 spark plug hole. Install leak-down tester adapter, pressurize to 100 PSI, record leak-down percentage, diagnose leak source, and document findings.
6. 6

   Test cylinder #3
   Continue rotating crankshaft clockwise following the firing order sequence. Bring cylinder #7 to TDC (next in firing order), then continue to cylinder #3 (which comes later in sequence). At cylinder #3 TDC compression stroke, install adapter, pressurize, record leak percentage, diagnose source, and document.
7. 7

   Test cylinder #4
   Rotate crankshaft to bring cylinder #6 to TDC, then continue to cylinder #5, then to cylinder #4. At cylinder #4 TDC compression stroke, install adapter, pressurize to 100 PSI, record leak-down percentage, diagnose leak source, and document findings.
8. 8

   Test cylinders #5 through #8
   Continue the process for remaining cylinders following the firing order 1-8-7-2-6-5-4-3. For each cylinder, rotate to TDC compression stroke, install adapter, pressurize to 100 PSI, record leak percentage (normal 5-10%, marginal 10-20%, problem >20%), diagnose leak source by listening at intake, exhaust, coolant, and crankcase, then document all findings. Ensure each cylinder is tested methodically.
9. 9

   Analyze test results
   Review all recorded leak-down percentages. Cylinders with readings above 20% require further investigation or repair. Compare readings between cylinders - variation greater than 5% between cylinders may indicate developing issues. If multiple adjacent cylinders show high leak-down with air escaping into coolant, suspect head gasket failure. If all cylinders show ring leakage, suspect worn piston rings. Single cylinder issues typically indicate valve problems.
10. 10

    Reinstall spark plugs
    If reusing spark plugs, verify gap is 0.040 inches and inspect threads for damage. If installing new plugs, verify correct gap specification. Apply a small amount of anti-seize compound to spark plug threads (avoiding electrode area). Thread each spark plug by hand into its respective cylinder to prevent cross-threading, then tighten to proper torque specification in a crisscross pattern across the engine.
11. 11

    Reinstall ignition coils
    Install each ignition coil into its correct cylinder location (using labels from preparation). Press coils firmly into spark plug wells ensuring full seating. Install and tighten the 10mm mounting bolt on each coil. Reconnect electrical connectors to each coil, ensuring they click into place securely.
    Torque spec

    Sensor Bolts14 Nm (10 lb-ft)
12. 12

    Reconnect battery and install engine cover
    Reconnect the negative battery cable and tighten securely. Reinstall the engine cover by aligning the grommets with mounting posts and pressing down firmly until seated. Verify all tools and equipment have been removed from engine compartment.

Reassembly

1. Ensure all eight ignition coil connectors are fully seated and locked
2. Verify engine cover is properly secured to all mounting grommets
3. Double-check that no tools or rags remain in engine compartment

Verification

• Start engine and verify it runs smoothly without misfires or rough idle
• Check for any vacuum leaks around spark plug wells or ignition coils
• Review leak-down test results: acceptable readings are 5-10%, marginal are 10-20%, and readings above 20% indicate repair is needed
• Compare leak-down percentages between cylinders - variation should be less than 5% for a healthy engine
• Use diagnostic findings to determine if valve adjustment, head gasket replacement, valve work, or engine rebuild is necessary based on where air was escaping during the test

More procedures for this vehicle