A leak-down test measures cylinder compression loss to diagnose internal engine problems such as worn piston rings, valve leaks, or head gasket failures on the 3.5L EcoBoost V6.

Warnings

Tools required

Parts

• Spark plugs (if worn) × 6 — Motorcraft SP-546 or equivalent

Preparation

1. Ensure engine is completely cold (below 100°F) to prevent thread damage
2. Park vehicle on level surface and engage parking brake
3. Disconnect negative battery terminal to prevent accidental starter engagement
4. Remove engine cover by pulling up on cover clips
5. Label all ignition coil connectors with cylinder numbers (1-6) for correct reinstallation

Procedure

1. 1

   Remove air intake system
   Release air box clips and remove the intake air box assembly to access the rear cylinder bank ignition coils. Disconnect the mass airflow sensor electrical connector. Remove the air intake tube from the throttle body.
   Torque spec

   Air Box Clips5 Nm (4 lb-ft)
2. 2

   Remove ignition coils from all cylinders
   Disconnect the electrical connector from each ignition coil. Remove the mounting bolt from each coil. Pull coils straight up with a twisting motion to remove. Keep coils organized by cylinder number (front bank driver side: 1-2-3, rear bank passenger side: 4-5-6).
   ⚠Ignition coils are fragile. Do not pry or use excessive force when removing.
   Torque spec

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

   Remove all spark plugs
   Using a 14mm spark plug socket with extension, carefully remove all six spark plugs. Inspect each plug for fouling, oil deposits, or unusual wear patterns. Keep plugs organized by cylinder number for diagnosis.
   ⚠Aluminum cylinder heads are easily damaged. Use proper spark plug socket and ensure it is fully seated before applying torque.
4. 4

   Rotate engine to TDC for cylinder 1
   Using a socket on the crankshaft pulley bolt (21mm), rotate the engine clockwise until cylinder 1 is at Top Dead Center (TDC) on the compression stroke. Both intake and exhaust valves should be closed. Verify by feeling for compression at the spark plug hole.
   ℹ️On the compression stroke, both valves are closed. If air escapes during rotation, continue rotating 360 degrees to the next compression cycle.
5. 5

   Connect leak-down tester to cylinder 1
   Thread the leak-down tester adapter into cylinder 1 spark plug hole by hand to ensure proper thread engagement. Connect the leak-down tester to the adapter and to the compressed air supply. Set regulator to 100 psi.
   ⚠If engine rotates when air is applied, piston is not at TDC. Remove air pressure and reposition crankshaft.
6. 6

   Perform leak-down test on cylinder 1
   Apply air pressure and read the leak-down percentage on the tester gauge. Listen at the tailpipe (exhaust valve leak), intake (intake valve leak), oil filler cap (piston ring leak), and radiator/overflow tank (head gasket leak). Record percentage and location of any leaks. Normal is 5-10%, acceptable is 10-15%, problematic is above 20%.
   ℹ️Some air escaping through PCV system is normal and does not indicate ring failure.
7. 7

   Test remaining cylinders
   Remove leak-down tester from cylinder 1. Rotate crankshaft clockwise to bring each cylinder to TDC on compression stroke in firing order (1-4-2-5-3-6). Repeat leak-down test procedure for cylinders 2, 3, 4, 5, and 6. Record all results with cylinder numbers.
   ℹ️Firing order for 3.5L EcoBoost is 1-4-2-5-3-6. Front bank (driver side) is 1-2-3, rear bank (passenger side) is 4-5-6.
8. 8

   Analyze test results
   Compare leak-down percentages across all cylinders. Variation greater than 5% between cylinders indicates a problem. Identify leak source based on where air was escaping: exhaust valve (air at tailpipe), intake valve (air at throttle body), piston rings (air at oil filler), head gasket (air in coolant or adjacent cylinder).
   ℹ️Document all findings including percentage and leak location for each cylinder before reassembly.
9. 9

   Reinstall spark plugs
   Apply a small amount of anti-seize to spark plug threads. Hand-thread each spark plug into its original cylinder location to prevent cross-threading. Torque spark plugs to 15 Nm (11 lb-ft) using torque wrench.
   ⚠Over-torquing spark plugs will strip aluminum threads. Use torque wrench and proper specification.
10. 10

    Reinstall ignition coils
    Install each ignition coil into its correct cylinder location using your labels from disassembly. Push coils down firmly until seated. Install mounting bolts and reconnect electrical connectors to each coil.
    Torque spec

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

    Reinstall air intake system
    Reinstall the air intake tube to the throttle body. Reconnect the mass airflow sensor electrical connector. Install the air box assembly and secure all clips.
    Torque spec

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

    Reconnect battery and install engine cover
    Reconnect the negative battery terminal. Install the engine cover by pressing down on all mounting points until clips engage. Ensure all tools and materials are removed from engine bay.

Reassembly

1. Verify all ignition coil electrical connectors are fully seated and locked
2. Ensure air intake system is properly secured with no loose connections
3. Double-check that all tools have been removed from engine compartment
4. Verify engine cover is properly installed and all clips are engaged

Verification

• Start engine and verify smooth idle with no misfires or rough running
• Check for any error codes related to ignition system using scan tool
• Verify no air leaks from intake system during engine operation
• Review leak-down test results: 5-10% is excellent, 10-15% is acceptable, 15-20% requires monitoring, above 20% requires engine repair
• If high leak-down was detected, schedule appropriate repairs based on leak source identified

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