A leak-down test measures the percentage of pressure loss in each cylinder to diagnose internal engine wear, valve sealing issues, and head gasket integrity on the 3.5L EcoBoost engine.

Warnings

Tools required

Parts

• Spark plugs (if found damaged during removal) × 6 — Motorcraft SP-546 or equivalent

Preparation

1. Park vehicle on level surface and engage parking brake firmly
2. Ensure engine is completely cold (not run for at least 2 hours)
3. Disconnect negative battery cable to prevent accidental starter engagement
4. Remove engine cover by pulling upward on cover - no fasteners required on most 2015-2020 models
5. Set up leak-down tester according to manufacturer instructions and verify calibration
6. Ensure shop air supply is regulated to 90-100 PSI and stable

Procedure

1. 1

   Remove ignition coils and spark plugs
   Disconnect all six ignition coil electrical connectors by pressing release tabs. Remove coil hold-down bolts and pull coils straight out with twisting motion. Use 18mm spark plug socket to remove all six spark plugs. Inspect plugs for fouling, oil, or unusual wear patterns that may indicate issues. Label plug locations for reference.
2. 2

   Prepare cylinder 1 for testing
   Locate cylinder 1 (front passenger side, forward-most cylinder). Rotate crankshaft clockwise using 36mm socket on crankshaft pulley bolt until cylinder 1 is approximately 15-20 degrees after top dead center (ATDC) on compression stroke. Both intake and exhaust valves should be fully closed. Verify by checking that rocker arms for cylinder 1 have slight clearance and are not under load.
3. 3

   Install leak-down tester on cylinder 1
   Thread the leak-down tester adapter into cylinder 1 spark plug hole hand-tight, then snug gently - do not over-tighten. Connect leak-down tester to adapter and shop air supply. Slowly introduce air pressure while monitoring that crankshaft does not rotate. If rotation occurs, adjust crankshaft position slightly past TDC.
   ⚠Apply air pressure gradually. Sudden pressure application can rotate the engine unexpectedly.
4. 4

   Perform cylinder 1 leak-down measurement
   With air pressure stabilized at tester's specified input (typically 100 PSI), read the percentage of leakage on the gauge. Record the percentage for cylinder 1. Listen at the tailpipe (exhaust valve leakage), throttle body/intake (intake valve leakage), oil filler cap (piston ring leakage), and cooling system overflow tank (head gasket leakage) to identify leak source. Document findings for each cylinder.
5. 5

   Test cylinder 2
   Release air pressure from cylinder 1 and remove tester adapter. Rotate crankshaft clockwise approximately 120 degrees to position cylinder 2 (front driver side, forward cylinder) 15-20 degrees ATDC on compression stroke. Install tester adapter, apply air pressure, and record leak-down percentage and leak sources.
6. 6

   Test cylinder 3
   Release air pressure and remove adapter. Rotate crankshaft clockwise approximately 120 degrees to position cylinder 3 (front passenger side, rear cylinder) 15-20 degrees ATDC on compression stroke. Install tester adapter, apply air pressure, and record leak-down percentage and leak sources.
7. 7

   Test cylinder 4
   Release air pressure and remove adapter. Rotate crankshaft clockwise approximately 120 degrees to position cylinder 4 (rear driver side, rear cylinder) 15-20 degrees ATDC on compression stroke. Install tester adapter, apply air pressure, and record leak-down percentage and leak sources.
8. 8

   Test cylinder 5
   Release air pressure and remove adapter. Rotate crankshaft clockwise approximately 120 degrees to position cylinder 5 (rear passenger side, rear cylinder) 15-20 degrees ATDC on compression stroke. Install tester adapter, apply air pressure, and record leak-down percentage and leak sources.
9. 9

   Test cylinder 6
   Release air pressure and remove adapter. Rotate crankshaft clockwise approximately 120 degrees to position cylinder 6 (rear driver side, forward cylinder) 15-20 degrees ATDC on compression stroke. Install tester adapter, apply air pressure, and record leak-down percentage and leak sources. This completes testing of all six cylinders.
10. 10

    Reinstall spark plugs and ignition coils
    Apply a small amount of anti-seize to spark plug threads if desired (avoid getting on electrodes). Install all six spark plugs hand-tight, then torque to 15 Nm (11 lb-ft) using spark plug socket. Insert ignition coils into spark plug tubes, install hold-down bolts, and reconnect all electrical connectors. Ensure coils are fully seated.
11. 11

    Reinstall engine cover and reconnect battery
    Position engine cover over engine and press downward until mounting grommets snap into place. Reconnect negative battery cable. Clear any diagnostic trouble codes if battery was disconnected for extended period.

Reassembly

1. Verify all ignition coil connectors are fully seated and locked
2. Ensure engine cover is properly secured at all mounting points
3. Double-check that all tools and equipment have been removed from engine bay

Verification

• Review leak-down test results: 5-10% is excellent, 10-15% is acceptable, above 15% indicates issues requiring diagnosis
• Compare results between cylinders - variation greater than 5% between cylinders may indicate specific cylinder problems
• Cross-reference leak source identification (intake, exhaust, rings, head gasket) with leak-down percentages for accurate diagnosis
• Start engine and verify smooth idle with no misfires - if misfires occur, verify spark plug installation and ignition coil connections
• If high leak-down was detected, recommend appropriate next steps: valve service for valve leakage, ring replacement for ring leakage, or head gasket replacement for gasket leakage

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