This procedure performs a compression test on all six cylinders of the 3.5L EcoBoost V6 engine to diagnose internal engine condition, including rings, valves, and head gasket integrity.

Warnings

Tools required

Parts

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

Preparation

1. Ensure engine is at normal operating temperature, then shut off and allow to cool for at least 2 hours
2. Park vehicle on level surface and engage parking brake
3. Disconnect negative battery terminal, then reconnect (this step is to prepare for fuel pump disable)
4. Locate fuel pump relay in engine compartment fuse box (typically relay R9) and remove it to disable fuel system
5. Remove engine cover by pulling upward on cover - it is retained by grommets
6. Label all ignition coil positions (cylinder 1-6) before removal to ensure correct reinstallation

Procedure

1. 1

   Remove air intake system components
   Disconnect the air intake tube from the throttle body. Release air box clips and remove the air box assembly to gain access to the right bank ignition coils. Set components aside in a clean area.
   Torque spec

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

   Disconnect ignition coils
   Disconnect the electrical connectors from all six ignition coils by pressing the release tab and pulling straight up. Left bank: cylinders 1, 2, 3 (front to rear). Right bank: cylinders 4, 5, 6 (front to rear).
3. 3

   Remove ignition coils
   Remove the mounting bolt from each ignition coil. Pull each coil straight up with a twisting motion to break the seal. Inspect coil boots for damage or oil contamination. Store coils in labeled order.
   Torque spec

   Mounting Bolts31 Nm (23 lb-ft)
4. 4

   Remove all spark plugs
   Using a 9/16 inch spark plug socket with extension, remove all six spark plugs. Turn counterclockwise and carefully extract each plug. Inspect plugs for fouling, oil, or abnormal wear patterns which may indicate engine problems. Keep plugs in order if they will be reinstalled.
   ⚠Debris falling into spark plug wells can cause severe engine damage. Use compressed air to clean around spark plug wells before removal if available.
5. 5

   Disable ignition system
   Locate and remove all three ignition coil fuses from the engine compartment fuse box to prevent ignition system activation during cranking. Typical fuses are F27, F28, and F29 (verify in owner's manual for specific year).
6. 6

   Test cylinder 1 compression
   Install compression gauge firmly into cylinder 1 spark plug hole (front left). Ensure throttle plate is fully open by having helper press accelerator pedal to floor. Crank engine for 5-6 compression strokes (about 3-4 seconds). Record the maximum pressure reading. Release gauge pressure valve.
   ℹ️Compression should build progressively with each stroke. Low initial compression that builds up indicates worn rings; low compression that stays low indicates valve or head gasket issues.
7. 7

   Test remaining cylinders
   Repeat compression test procedure for cylinders 2 through 6 in order. For each cylinder: install gauge, open throttle fully, crank for 5-6 strokes, record maximum pressure, and release gauge. Allow 15-30 seconds between tests to prevent starter overheating.
8. 8

   Analyze compression results
   Compare all cylinder readings. Normal range is 150-200 psi. Calculate variance: subtract lowest reading from highest, divide by highest, multiply by 100. Variance should be under 15%. If any cylinder is more than 15% low, perform wet compression test by adding 1 tablespoon of oil to that cylinder and retesting.
   ℹ️If wet test significantly improves compression, rings are worn. If wet test shows no improvement, suspect valve or head gasket issues.
9. 9

   Reinstall spark plugs
   Apply small amount of anti-seize compound to spark plug threads (avoid getting any on electrodes). Hand-thread each spark plug carefully into its original cylinder position. Torque spark plugs to 15 Nm (11 lb-ft) using torque wrench. Do not over-tighten as aluminum heads can strip easily.
10. 10

    Reinstall ignition coils
    Install each ignition coil into its labeled position, pushing down firmly until seated. Install mounting bolt for each coil and torque to specification. Reconnect all six ignition coil electrical connectors until they click.
    Torque spec

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

    Restore fuel and ignition systems
    Reinstall fuel pump relay into its original position in the fuse box. Reinstall all three ignition coil fuses. Ensure all fuses and relays are fully seated.
12. 12

    Reinstall air intake components
    Reinstall air box assembly and secure with clips. Reconnect air intake tube to throttle body. Reinstall engine cover by aligning grommets and pressing down firmly until seated.
    Torque spec

    Air Box Clips5 Nm (4 lb-ft)

Reassembly

1. Verify all ignition coil connectors are fully engaged and locked
2. Confirm all fuses and relays have been reinstalled in the fuse box
3. Check that no tools or parts have been left in the engine compartment

Verification

• Start engine and verify it runs smoothly without misfires or rough idle
• Check for any diagnostic trouble codes using an OBD-II scanner
• Listen for any abnormal engine noises that may indicate improperly seated components
• Review compression test results: all cylinders should be within 150-200 psi range with less than 15% variance between highest and lowest readings

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