A leak-down test measures the percentage of air leakage past the piston rings, valves, and head gasket to assess engine internal condition and identify cylinder sealing problems.

Warnings

Tools required

Parts

• Spark plugs (if replacement needed) × 4 — Use OEM specification

Preparation

1. Ensure engine is completely cold to touch
2. Park vehicle on level ground and engage parking brake
3. Open hood and allow engine bay to cool if recently operated
4. Disconnect negative battery terminal to disable ignition and fuel systems
5. Remove engine cover by pulling upward on four corner clips
6. Label and photograph ignition coil connections for reassembly reference

Procedure

1. 1

   Remove ignition coils and spark plugs
   Disconnect the four ignition coil electrical connectors by pressing release tabs. Remove the coil mounting bolts and carefully pull coils straight up from spark plug wells. Using a 16mm spark plug socket, remove all four spark plugs and inspect condition. Keep plugs in order by cylinder number (1-4, left to right facing engine).
2. 2

   Disable fuel system
   Locate the fuel pump fuse in the engine bay fuse box (typically fuse #17, 15A). Remove the fuse to prevent fuel delivery during testing. Alternatively, disconnect the fuel pump relay if preferred.
3. 3

   Bring cylinder 1 to TDC compression stroke
   Insert a long screwdriver or piston stop tool into cylinder 1 spark plug hole. Using a breaker bar on the crankshaft pulley bolt (22mm), slowly rotate the engine clockwise while feeling for the piston to rise. When piston reaches the top and stops rising, confirm both intake and exhaust valves are closed by observing valve cover (no rocker movement). This is TDC compression stroke.
4. 4

   Install leak-down tester on cylinder 1
   Thread the leak-down tester adapter fitting into cylinder 1 spark plug hole by hand, then snug with the spark plug socket (do not overtighten). Connect the regulated compressed air supply to the tester input port. Verify regulator is set to 80-100 PSI as specified by your tester manufacturer.
5. 5

   Perform leak-down test on cylinder 1
   Slowly apply air pressure while observing the gauge. Record both input and output gauge readings (percentage of leakage). With air applied, listen at the intake, exhaust, oil fill cap, and adjacent cylinders to identify leak source. Intake hissing indicates intake valve leakage, exhaust indicates exhaust valve leakage, oil fill cap indicates piston ring leakage, and adjacent cylinder indicates head gasket leakage. Record all findings including percentage and leak location.
6. 6

   Test cylinder 2
   Release air pressure and remove tester from cylinder 1. Rotate crankshaft approximately 180 degrees clockwise to bring cylinder 2 to TDC compression stroke (verify both valves closed). Install leak-down tester into cylinder 2, apply air pressure, and record readings and leak locations as performed on cylinder 1.
7. 7

   Test cylinder 3
   Release air pressure and remove tester. Rotate crankshaft approximately 180 degrees clockwise to bring cylinder 3 to TDC compression stroke. Install tester, perform leak-down test, and record all results including percentage and leak source locations.
8. 8

   Test cylinder 4
   Release air pressure and remove tester. Rotate crankshaft approximately 180 degrees clockwise to bring cylinder 4 to TDC compression stroke. Install tester, perform final leak-down test, and record complete results. Compare all four cylinders for consistency.
9. 9

   Analyze test results
   Review all recorded data. Normal leakage is 5-10% per cylinder. Values of 10-15% indicate minor wear. Values of 15-20% indicate moderate wear requiring monitoring. Values above 20% indicate significant problems requiring repair. Note any cylinder with readings significantly different from others (more than 5% variance). Document leak sources identified during testing to determine required repairs (valve job, rings, head gasket, etc.).
10. 10

    Reinstall spark plugs
    Inspect spark plugs for fouling, wear, or damage. Replace if gap exceeds specification or electrodes show excessive wear. Thread spark plugs into cylinder head by hand to prevent cross-threading. Torque to 25-30 Nm (18-22 lb-ft) using spark plug socket and torque wrench.
11. 11

    Reinstall ignition coils
    Apply small amount of dielectric grease to spark plug terminal ends if desired. Install ignition coils into spark plug wells, pressing down firmly until seated. Install coil mounting bolts and torque to specification. Reconnect all four ignition coil electrical connectors ensuring they click into place.
    Torque spec

    Sensor Bolts10 Nm (8 lb-ft)
12. 12

    Restore vehicle systems
    Reinstall fuel pump fuse or relay. Reinstall engine cover by aligning clips and pressing down until all four corners snap into place. Reconnect negative battery terminal. Clear any diagnostic trouble codes that may have set during testing using scan tool if available.

Reassembly

1. Double-check all ignition coil connectors are fully seated and locked
2. Verify engine cover is properly clipped in all four corners
3. Ensure no tools or parts are left in engine bay

Verification

• Start engine and verify smooth idle with no misfires
• Listen for any unusual noises that were not present before testing
• Verify all leak-down percentages were recorded for each cylinder with leak source locations
• Compare results to baseline acceptable values: 5-10% normal, above 20% requires repair
• If excessive leakage found, plan appropriate repairs based on identified leak sources (valves, rings, or head gasket)

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