A leak-down test diagnoses internal engine condition by pressurizing each cylinder and measuring air loss to identify worn rings, valves, or head gasket issues.

Warnings

Tools required

Parts

• Spark plug anti-seize compound × 1 — Use high-temperature anti-seize
• Spark plugs (if worn or damaged) × 8 — Champion J-10Y or equivalent

Preparation

1. Park vehicle on level surface and engage parking brake firmly
2. Ensure engine is completely cool to the touch
3. Disconnect negative battery cable to prevent accidental starter engagement
4. Remove air cleaner assembly and note vacuum line routing
5. Label all spark plug wires with cylinder number using masking tape before removal
6. Set air compressor regulator to 100 PSI and verify leak-down tester calibration

Procedure

1. 1

   Remove all spark plugs
   Disconnect all spark plug wires by pulling on the boot, not the wire. Using a 5/8 inch spark plug socket, remove all eight spark plugs in firing order sequence (1-8-4-3-6-5-7-2). Keep plugs organized by cylinder for condition inspection. Examine plug electrodes for abnormal wear patterns that may correlate with leak-down results.
2. 2

   Position cylinder #1 at Top Dead Center compression stroke
   Rotate engine by turning the harmonic balancer bolt clockwise with a 1-1/8 inch socket until the timing mark on the balancer aligns with the '0' mark on the timing tab. Verify cylinder #1 is on compression stroke by feeling for air pressure from spark plug hole or confirming both valves are closed. The AMC 390 firing order is 1-8-4-3-6-5-7-2.
3. 3

   Install piston stop in cylinder #1
   Thread the piston stop tool or insert approximately 18 inches of soft rope into cylinder #1 spark plug hole. This prevents the piston from being forced down when air pressure is applied. Ensure the stop is secure but do not over-tighten threaded stops to avoid cross-threading the aluminum heads.
4. 4

   Connect leak-down tester to cylinder #1
   Thread the leak-down tester adapter firmly into cylinder #1 spark plug hole. Connect the air supply hose to the tester. Slowly introduce air pressure while watching both gauges. The input gauge should read 100 PSI and the output gauge will show the percentage of leakage. Record the percentage reading after the gauges stabilize (5-10 seconds).
   ⚠Introduce air pressure gradually. Sudden pressurization can cause engine to rotate violently if piston stop is not properly seated.
5. 5

   Identify leak-down path for cylinder #1
   While maintaining pressure, listen at the carburetor intake (intake valve leak), tailpipe (exhaust valve leak), oil filler cap or dipstick tube (piston ring leak), and adjacent spark plug holes (head gasket leak). A hissing sound indicates the leak path. Record observations for cylinder #1. Acceptable leak-down is 5-10%, marginal is 10-20%, poor is 20-30%, and above 30% indicates major wear requiring repair.
6. 6

   Test cylinder #2
   Release air pressure and remove leak-down tester and piston stop from cylinder #1. Rotate crankshaft clockwise approximately 90 degrees to bring cylinder #8 to TDC compression (next in firing order). Install piston stop, connect leak-down tester, pressurize, record leakage percentage, and identify leak path. Document all findings.
7. 7

   Test remaining cylinders in firing order
   Continue testing cylinders in firing order sequence (4-3-6-5-7-2), rotating crankshaft 90 degrees between each test. For the AMC 390, each cylinder is 90 degrees apart in rotation. Maintain consistent 100 PSI input pressure for each cylinder. Record leakage percentage and leak paths for all eight cylinders. Look for patterns: multiple cylinders with high leakage may indicate general wear, while one cylinder indicates localized damage.
8. 8

   Analyze test results
   Compare all cylinder readings. Variance of more than 10% between cylinders indicates uneven wear. Correlate leak-down results with spark plug condition observed earlier. If excessive leakage is through the carburetor or tailpipe, valve work is needed. If leakage is at the crankcase (oil filler/dipstick), piston rings or cylinder walls are worn. If air exits an adjacent cylinder, head gasket failure is likely between those cylinders.

Reassembly

1. Clean spark plug threads in cylinder heads with compressed air to remove any debris
2. Apply small amount of anti-seize compound to spark plug threads (avoid getting compound on electrodes)
3. Install all eight spark plugs hand-tight, then snug with spark plug socket using moderate torque (25-30 ft-lbs typical for AMC engines with aluminum heads)
4. Reconnect spark plug wires to correct cylinders according to firing order, ensuring boots snap firmly onto plugs
5. Reinstall air cleaner assembly and reconnect all vacuum lines to original positions
6. Reconnect negative battery cable

Verification

• Review all recorded leak-down percentages to ensure complete data was collected from all eight cylinders
• Verify all spark plugs are properly torqued and spark plug wires are correctly routed
• If leak-down results are acceptable (under 15% on all cylinders), engine internal condition is good
• If high leak-down was detected, use recorded leak paths to diagnose specific repairs needed: valve service for intake/exhaust leaks, ring replacement for crankcase leaks, or head gasket replacement for inter-cylinder leaks
• Start engine briefly to confirm proper reconnection of all components and normal operation

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