This procedure performs a compression test on all six cylinders of the 3.5L EcoBoost V6 engine to diagnose engine mechanical condition and identify potential issues with valves, piston rings, or head gaskets.

Warnings

Tools required

Parts

• Spark plugs (if reusing old plugs over 30k miles) × 6 — Motorcraft SP-546 or equivalent iridium

Preparation

1. Warm engine to full operating temperature (approximately 195°F coolant temp), then shut off and allow to cool for 5-10 minutes to avoid burn risk
2. Park vehicle on level surface, engage parking brake, and place wheel chocks
3. Open hood and install fender covers to protect paint
4. Disconnect negative battery terminal to prevent accidental starting during preparation
5. Label and photograph ignition coil connections for correct reinstallation

Procedure

1. 1

   Remove engine cover
   Grasp the engine cover firmly at the front edges and pull upward to disengage the retaining grommets. Lift the cover away from the engine bay and set aside.
2. 2

   Remove ignition coils
   Disconnect the electrical connector from each ignition coil by pressing the release tab and pulling straight up. Remove the mounting bolt from each coil, then carefully pull each coil straight up out of the spark plug well. Use coil puller tool if coils are difficult to remove. Keep coils organized by cylinder position.
   ⚠Do not twist or rock ignition coils during removal as this can damage the spark plug boots.
   Torque spec

   Connector Bolts8 Nm (6 lb-ft)
3. 3

   Remove all spark plugs
   Using the 16mm spark plug socket with extension, carefully remove all six spark plugs. Turn counterclockwise and maintain straight alignment to avoid cross-threading. Inspect each plug and note its cylinder location. Check for oil fouling, abnormal wear, or damage that may indicate engine problems.
   ℹ️Spark plug condition can provide diagnostic clues: oil fouling suggests ring/valve seal issues, while white deposits may indicate lean mixture or coolant intrusion.
4. 4

   Disable fuel system
   Remove the fuel pump fuse (fuse 27, 20A) from the battery junction box located in the engine compartment on the driver's side fenderwell. This prevents fuel delivery during cranking.
5. 5

   Prepare compression gauge
   Install the M12 x 1.25 threaded adapter onto the compression gauge. Ensure the gauge is zeroed or reset. Verify the hose connection is secure and the gauge valve operates correctly.
6. 6

   Test cylinder 1 (passenger side front)
   Thread the compression gauge adapter into cylinder 1 spark plug hole by hand until finger-tight, then snug gently with a wrench. Reconnect battery negative terminal. Fully depress the accelerator pedal to open the throttle body. Have assistant crank the engine for 5-6 compression strokes (approximately 3-4 seconds) or use remote starter switch. Record the maximum PSI reading. Release gauge pressure valve to reset.
   ⚠Throttle must be held wide open during testing to ensure unrestricted airflow for accurate readings.
7. 7

   Test cylinder 2 (passenger side middle)
   Remove compression gauge from cylinder 1. Thread gauge adapter into cylinder 2 spark plug hole. Ensure throttle is wide open and crank engine for 5-6 compression strokes. Record maximum PSI reading and compare to cylinder 1. Reset gauge.
8. 8

   Test cylinder 3 (passenger side rear)
   Move compression gauge to cylinder 3. Thread adapter into spark plug hole, open throttle fully, and crank engine for 5-6 strokes. Record PSI reading and reset gauge.
9. 9

   Test cylinder 4 (driver side front)
   Move compression gauge to cylinder 4 on the driver side. Thread adapter into spark plug hole, open throttle fully, and crank engine for 5-6 strokes. Record PSI reading and reset gauge.
10. 10

    Test cylinder 5 (driver side middle)
    Move compression gauge to cylinder 5. Thread adapter into spark plug hole, open throttle fully, and crank engine for 5-6 strokes. Record PSI reading and reset gauge.
11. 11

    Test cylinder 6 (driver side rear)
    Move compression gauge to cylinder 6. Thread adapter into spark plug hole, open throttle fully, and crank engine for 5-6 strokes. Record final PSI reading. Remove compression gauge and adapter.
12. 12

    Reinstall spark plugs
    Inspect spark plug threads and electrode condition. Apply small amount of anti-seize compound to plug threads if installing new plugs (avoid getting anti-seize on electrodes). Thread each spark plug by hand into its original cylinder location to prevent cross-threading. Tighten to manufacturer specification using torque wrench. For used plugs: 15-18 Nm (11-13 lb-ft). For new plugs: 18-22 Nm (13-16 lb-ft).
    ⚠Over-torquing spark plugs can strip aluminum cylinder head threads. Use torque wrench for proper installation.
13. 13

    Reinstall ignition coils
    Apply thin coat of dielectric grease to inside of each coil boot. Align each coil with its spark plug and press down firmly until fully seated. Install mounting bolt and tighten. Reconnect electrical connector to each coil, ensuring it clicks into place.
    Torque spec

    Connector Bolts8 Nm (6 lb-ft)
14. 14

    Restore fuel system and test
    Reinstall fuel pump fuse (fuse 27). Disconnect battery negative terminal again. Reinstall engine cover by aligning grommets and pressing down until seated. Reconnect battery negative terminal. Turn ignition to ON position (without starting) for 5 seconds, then OFF. Repeat twice to prime fuel system. Start engine and verify normal operation with no misfires.

Reassembly

1. Ensure all ignition coil connectors are fully seated and locked
2. Verify engine cover is properly seated on all mounting grommets
3. Check for any tools or parts left in engine bay before closing hood

Verification

• Compare all cylinder compression readings: should be 140-170 PSI with no more than 15% variation between highest and lowest cylinders
• If any cylinder is more than 15% lower: perform wet compression test by adding 1 tablespoon of engine oil to suspect cylinder and retesting. Significant increase indicates worn rings; no change suggests valve or head gasket issues
• Start engine and verify smooth idle with no misfires or check engine light
• Ensure no fuel odor indicating fuel system leaks after reconnection

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