A leak-down test pressurizes each cylinder individually to measure compression loss and diagnose internal engine wear, valve seal issues, or head gasket failure on the 3.5L EcoBoost twin-turbo V6.

Warnings

Tools required

Parts

• Spark plugs (if reusing plugs over 60k miles) × 6 — Motorcraft SP-546 or equivalent iridium

Preparation

1. Park vehicle on level surface, engage parking brake, and place wheel chocks
2. Allow engine to cool to ambient temperature (minimum 2 hours after running)
3. Disconnect negative battery terminal to prevent accidental starter engagement
4. Remove engine cover by pulling upward on cover clips
5. Relieve fuel system pressure by removing fuel pump fuse and cranking engine until it stalls
6. Set up compressed air source and verify regulator is set to 100 PSI
7. Label all ignition coils with cylinder number before removal to ensure correct reinstallation

Procedure

1. 1

   Remove air intake system
   Disconnect mass airflow sensor electrical connector. Release air box clips and remove complete air intake tube assembly from throttle body to air filter housing. Set aside to provide access to front cylinder bank.
2. 2

   Remove ignition coils from all cylinders
   Disconnect electrical connectors from all six ignition coils. Remove the single mounting bolt from each coil. Use coil puller or twist gently while pulling upward to extract coils from spark plug wells. Inspect coil boots for cracks or oil contamination.
   Torque spec

   Mounting Bolts31 Nm (23 lb-ft)
3. 3

   Remove all spark plugs
   Using 16mm spark plug socket, remove all six spark plugs. Keep plugs organized by cylinder number for inspection. Examine plugs for oil fouling, carbon buildup, or electrode erosion that may indicate cylinder-specific issues. Clean debris from spark plug wells with compressed air before proceeding.
   ⚠Use low-pressure air only to clean plug wells. Debris forced into cylinders can cause severe engine damage.
4. 4

   Rotate cylinder 1 to TDC compression stroke
   Remove radiator cap to eliminate compression resistance. Rotate crankshaft clockwise (viewed from front) using breaker bar on harmonic balancer bolt until cylinder 1 reaches Top Dead Center on compression stroke. Verify by confirming timing marks align and both intake and exhaust valves are closed. You can feel resistance when both valves are seated.
   ℹ️On 3.5L EcoBoost, cylinder 1 is front passenger side. Bank 1: cylinders 1-2-3 (passenger), Bank 2: cylinders 4-5-6 (driver).
5. 5

   Install leak-down tester on cylinder 1
   Thread leak-down tester adapter into cylinder 1 spark plug hole by hand, then tighten securely. Install harmonic balancer holding tool or piston stop to prevent crankshaft rotation when air pressure is applied. Connect leak-down tester supply hose to compressed air source and gauge hose to cylinder adapter.
   ⚠Engine will try to rotate when air pressure is applied if not properly secured. Ensure holding tool is firmly installed.
6. 6

   Perform leak-down test on cylinder 1
   Slowly apply air pressure while observing both gauges. Supply gauge should read 100 PSI, cylinder gauge shows percentage of pressure retained. Record cylinder pressure percentage. Listen at oil filler cap (piston rings), throttle body (intake valves), and exhaust pipe (exhaust valves) to identify leak source. Normal reading is 5-10% leak, marginal is 10-20%, poor is over 20%. Adjacent cylinder leakage indicates head gasket failure.
7. 7

   Test remaining cylinders in sequence
   Release air pressure and remove tester from cylinder 1. Rotate crankshaft to bring next cylinder to TDC on compression stroke (firing order: 1-4-2-5-3-6, equals 120° rotation between cylinders). Repeat installation and testing procedure for cylinders 4, 2, 5, 3, and 6 in sequence. Record results for each cylinder including leak percentage and leak source location.
   ℹ️On V6 engines, rotate crankshaft exactly 120° between tests. Mark balancer with chalk to track rotation if timing marks are not visible.
8. 8

   Analyze test results
   Compare leak percentages across all cylinders. Variation greater than 10% between cylinders indicates a problem. Identify leak sources: hissing at oil filler = worn rings, hissing at throttle = intake valve issue, hissing at exhaust = exhaust valve issue, hissing at adjacent cylinder = head gasket leak. Coolant bubbling in radiator during test indicates head gasket or crack.
   ℹ️EcoBoost engines with direct injection may show slightly higher leak-down percentages than port-injected engines due to valve design. Up to 15% can be acceptable if consistent across cylinders.
9. 9

   Reinstall spark plugs
   Inspect spark plug threads and apply small amount of anti-seize to threads (avoid electrode end). Install plugs hand-tight, then torque to specification. EcoBoost engines use torque-to-yield spark plugs requiring precise torque. Do not over-torque as this can damage aluminum cylinder head threads.
   ⚠2011-2014 EcoBoost engines are prone to spark plug blowout if plugs are under-torqued. Follow factory torque specification exactly.
10. 10

    Reinstall ignition coils
    Verify each ignition coil boot is clean and undamaged. Install coils into correct cylinder positions matching original labels. Press coils fully into spark plug wells until seated. Install mounting bolts and torque to specification. Reconnect all coil electrical connectors with positive click.
    Torque spec

    Mounting Bolts31 Nm (23 lb-ft)
11. 11

    Reinstall air intake system
    Position air intake tube assembly and secure air box clips. Reconnect mass airflow sensor electrical connector. Ensure all clamps are tight and no vacuum leaks exist.
    Torque spec

    Air Box Clips5 Nm (4 lb-ft)
12. 12

    Reinstall engine cover and reconnect battery
    Install engine cover by aligning mounting grommets and pressing down until clips engage. Reconnect negative battery terminal. Reinstall fuel pump fuse to restore fuel system operation.

Reassembly

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

Verification

• Start engine and verify smooth idle with no misfires or rough running
• Check for proper ignition coil seating by listening for clicking or arcing sounds
• Review leak-down test data: readings within 5-10% indicate good compression, 10-20% marginal, over 20% requires repair
• Compare cylinder-to-cylinder variation: less than 10% difference is acceptable
• If high leakage detected, correlate with leak source to determine repair path (valve job, rings, head gasket)
• Clear any diagnostic trouble codes that may have set during testing

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