Perform a compression test on all four cylinders to diagnose engine mechanical condition and identify potential issues with rings, valves, or head gasket.

Warnings

Tools required

Parts

• Spark plugs (if worn or fouled) × 4 — 18855-10080 or equivalent

Preparation

1. Warm engine to full operating temperature (coolant at 180-195°F), then shut off and allow 10 minutes to cool slightly
2. Park vehicle on level surface and engage parking brake
3. Ensure battery is fully charged or connect battery charger/maintainer
4. Turn ignition off and remove key
5. Allow engine to cool enough to safely touch ignition coil area (approximately 10-15 minutes)

Procedure

1. 1

   Remove engine cover and air intake
   Remove the plastic engine cover by pulling upward on the cover to release the retaining grommets. Disconnect the air intake tube from the throttle body by loosening the hose clamp. Release the clips securing the air box lid and remove the air filter. Disconnect any breather hoses attached to the air box. Remove air box assembly to provide access to ignition coils.
2. 2

   Disconnect ignition coils
   Locate all four ignition coil packs mounted directly above the spark plugs. Press the tab and disconnect the electrical connector from each coil pack. Label each connector position (cylinders 1-4 from left to right facing engine) to ensure correct reassembly. Remove the mounting bolt from each coil pack using a 10mm socket.
   Torque spec

   Sensor Bolts10 Nm (8 lb-ft)
3. 3

   Remove ignition coils
   Carefully pull each ignition coil straight up and out of the spark plug well. Inspect the coil boot for cracks or carbon tracking. Set coils aside in order. Inspect spark plug wells for oil or coolant contamination, which indicates a separate issue requiring repair before testing.
4. 4

   Remove all spark plugs
   Using a 16mm spark plug socket with extension, carefully remove all four spark plugs. Keep plugs in order to identify cylinder-specific issues. Inspect each plug for fouling, oil deposits, or abnormal wear patterns which provide diagnostic information. Place clean shop towels loosely over the open spark plug holes to prevent debris entry.
   ⚠Do not allow any debris to fall into the open cylinder bores as this can cause severe engine damage.
5. 5

   Disable fuel system
   Locate the fuel pump fuse in the engine compartment fuse box (typically labeled as 'FUEL PUMP' or 'EFI'). Remove this fuse to disable fuel delivery during cranking. Alternatively, disconnect the fuel pump relay. This prevents raw fuel from being injected during the compression test which can wash cylinder walls and create a fire hazard.
6. 6

   Test cylinder 1 compression
   Remove the shop towel from cylinder 1 (leftmost cylinder). Thread the compression tester hose or adapter firmly into the spark plug hole by hand. Ensure a tight seal. Have an assistant or use a remote starter switch to crank the engine for 5-6 compression strokes (approximately 3-4 seconds) while holding the throttle fully open. Record the maximum PSI reading. Release pressure from gauge.
   ℹ️Throttle must be held wide open during all compression tests to allow maximum airflow into cylinders.
7. 7

   Test remaining cylinders
   Repeat the compression test procedure for cylinders 2, 3, and 4 in sequence. Record each reading. Ensure consistent cranking duration for each cylinder (5-6 compression strokes). Compare all readings. All cylinders should be within 15 PSI of each other and within the 170-200 PSI range for a healthy engine.
8. 8

   Perform wet test if needed
   If any cylinder shows low compression (more than 15 PSI below others or below 150 PSI), perform a wet test on that cylinder. Add approximately 1 tablespoon of clean engine oil through the spark plug hole into the cylinder. Retest compression. If reading increases significantly (30+ PSI), rings or cylinder walls are worn. If reading stays the same, suspect valve or head gasket issues.
   ℹ️This step only applies if low compression is detected during initial testing.
9. 9

   Reinstall spark plugs
   Ensure spark plug well threads are clean and free of debris or oil. Apply a small amount of anti-seize compound to the spark plug threads if desired (avoid getting any on electrodes). Install each spark plug by hand first to avoid cross-threading, then tighten to specification. Use a torque wrench to ensure proper seating without damaging threads or plugs. Install plugs in the correct cylinders if reusing old plugs.
   ⚠Over-torquing spark plugs can strip aluminum cylinder head threads requiring costly repair. Under-torquing can cause plugs to blow out.
10. 10

    Reinstall ignition coils
    Apply a small amount of dielectric grease inside each coil boot to prevent moisture intrusion and ensure good spark transfer. Push each ignition coil firmly into its corresponding spark plug well until fully seated. Install and tighten the mounting bolt on each coil pack. Reconnect the electrical connector to each coil, ensuring the locking tab clicks into place.
    Torque spec

    Sensor Bolts10 Nm (8 lb-ft)
11. 11

    Restore fuel system and reinstall components
    Reinstall the fuel pump fuse or relay that was removed earlier. Reinstall the air box assembly, reconnecting any breather hoses. Install the air filter and secure the air box lid with clips. Reconnect the air intake tube to the throttle body and tighten the hose clamp. Reinstall the engine cover by aligning the grommets and pressing down firmly until seated.

Reassembly

1. Double-check all electrical connectors are fully seated and locked
2. Verify air intake system is completely reassembled with no loose hoses
3. Confirm fuel pump fuse or relay has been reinstalled

Verification

• Turn ignition to ON position without starting engine and listen for fuel pump priming (2-3 second hum)
• Start engine and verify smooth idle with no misfires or rough running
• Check for any vacuum leaks around air intake connections
• Verify no check engine light related to ignition system
• Compare compression readings: all cylinders should be within 15 PSI of each other and between 170-200 PSI for healthy engine
• If compression is low or uneven, diagnose based on wet test results and consider valve adjustment, head gasket replacement, or internal engine repair

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