A leak-down test pressurizes each cylinder individually to measure air leakage past rings, valves, and head gasket, helping diagnose engine internal condition without disassembly.

Warnings

Tools required

Parts

• Spark plug set (8 required) × 8 — Motorcraft SP-515 or equivalent

Preparation

1. Ensure engine is completely cold (below 100°F). Do not perform on warm engine.
2. Park vehicle on level surface, set parking brake, and place in Park position.
3. Disconnect negative battery cable to prevent accidental starter engagement.
4. Remove engine cover by pulling upward on front tabs and lifting off.
5. Locate and identify all 8 spark plug locations on both cylinder banks.
6. Have compressed air source ready with minimum 90 PSI capacity and leak-down tester calibrated.
7. Prepare notebook with cylinder numbering chart (passenger side 1-4 front to rear, driver side 5-8 front to rear).

Procedure

1. 1

   Remove ignition coils and spark plugs
   Disconnect all 8 ignition coil electrical connectors by pressing release tabs. Remove coil hold-down bolts using 7mm socket. Pull coils straight up from spark plug wells. Use spark plug socket with extension to remove all 8 spark plugs, keeping them in order for inspection. Inspect plug condition and gap for reference.
2. 2

   Rotate engine to TDC compression stroke for cylinder 1
   Using 24mm socket on crankshaft pulley bolt, rotate engine clockwise (viewed from front) until cylinder 1 piston approaches top dead center. Install piston stop tool in cylinder 1 spark plug hole. Carefully rotate crankshaft until piston contacts stop, then back off slightly. Remove stop and rotate forward gently until approximately 10 degrees ATDC compression stroke. Verify by checking that both intake and exhaust valves are closed (use flashlight to observe valve position through plug hole if needed).
3. 3

   Perform leak-down test on cylinder 1
   Set leak-down tester regulator to 100 PSI input pressure. Thread leak-down tester adapter into cylinder 1 spark plug hole hand-tight. Apply air pressure slowly while preventing engine rotation (have assistant apply brake or use transmission lock). Once cylinder is pressurized and crankshaft is locked by pressure, observe gauge reading. Record percentage of leakage shown on leak-down gauge. Listen at oil fill cap (rings), throttle body (intake valves), exhaust tailpipe (exhaust valves), and radiator cap (head gasket) to identify leak source. Record all findings including leak percentage and location of air escape.
4. 4

   Test cylinder 2
   Release air pressure from cylinder 1 and remove tester. Rotate crankshaft clockwise approximately 90 degrees to bring cylinder 2 to TDC compression stroke (firing order is 1-6-5-10-2-7-3-8 on Ford Coyote, so next in sequence is cylinder 6, but test sequentially by bank position). Use piston stop method to verify TDC compression stroke. Repeat leak-down test procedure, recording percentage and leak location. Compare result to cylinder 1 for pattern analysis.
5. 5

   Test cylinder 3
   Release pressure and remove tester from cylinder 2. Rotate crankshaft to bring cylinder 3 to TDC compression stroke. Verify both valves closed using piston stop method. Install leak-down tester and pressurize to 100 PSI. Record leakage percentage and identify leak paths by listening at oil fill, throttle body, exhaust, and radiator. Document all findings.
6. 6

   Test cylinder 4
   Continue testing cylinder 4 using same procedure: rotate to TDC compression, verify valve closure, pressurize to 100 PSI, record leakage percentage and source. This completes passenger side cylinder bank testing.
7. 7

   Test cylinders 5 through 8
   Repeat testing procedure for driver side cylinders 5, 6, 7, and 8 in sequence. For each cylinder: rotate crankshaft to TDC compression stroke, verify valve closure with piston stop, install tester, pressurize to 100 PSI, record leakage percentage and identify leak source by listening at oil fill cap, throttle body, exhaust pipe, and radiator. Maintain consistent 100 PSI input pressure for all tests to ensure comparable results.
8. 8

   Analyze test results
   Review recorded data for all 8 cylinders. Good cylinders typically show 5-10% leakage, acceptable range is up to 15%, concerning is 15-20%, and poor is above 20%. Compare cylinders for patterns: if one cylinder significantly worse, indicates localized problem; if all cylinders similar and high, indicates general wear; if adjacent cylinders both leak at head gasket area, indicates head gasket failure between those cylinders. Note any cylinders requiring further diagnosis.
9. 9

   Reinstall spark plugs
   If original plugs are in good condition (proper gap, minimal electrode wear, no fouling), apply small amount of anti-seize to threads and reinstall. If plugs show wear or vehicle is near service interval, install new spark plugs. Thread plugs in by hand to avoid cross-threading, then tighten to specification. For Motorcraft SP-515 plugs, torque is typically 15-18 lb-ft (not listed in provided specs, follow spark plug manufacturer specification on box).
10. 10

    Reinstall ignition coils
    Install ignition coils into spark plug wells, ensuring they seat fully onto spark plug terminals. Install coil hold-down bolts and tighten securely (approximately 7-9 lb-ft, hand-tight plus 1/4 turn if torque spec not available). Reconnect all 8 coil electrical connectors until they click into place. Verify each connection is secure.

Reassembly

1. Reinstall engine cover by aligning rear tabs first, then pressing down on front clips until secured.
2. Reconnect negative battery cable and torque to 35 lb-ft if equipped with threaded terminal.
3. Clear any diagnostic trouble codes that may have been set from cranking with coils disconnected.
4. Double-check that all ignition coil connectors are fully seated and all tools are removed from engine bay.

Verification

• Start engine and verify smooth idle with no misfires. Engine may run rough momentarily while fuel trims stabilize.
• Check that all 8 cylinders are firing by listening for even exhaust note and smooth engine operation.
• Review leak-down test results: cylinders with less than 15% leakage are acceptable, 15-20% requires monitoring, above 20% indicates repair needed.
• Compare results across cylinders to identify patterns: significant variation between cylinders indicates specific component failure rather than normal wear.
• If multiple cylinders show high leakage with air escaping at oil fill cap (rings), engine may need overhaul. If air escapes at exhaust or intake (valves), cylinder head service may be required. If air escapes at radiator (head gasket), head gasket replacement is needed.

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