Compression Test

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

Warnings

Tools required

Parts

• Spark plug gaskets (if reusing old plugs) × 8 — Use OEM specification

Preparation

1. Ensure battery is fully charged or connect battery charger to maintain voltage during multiple cranking cycles
2. Allow engine to reach operating temperature if possible, then shut off and allow 10-15 minutes cooling time
3. Disable fuel delivery by removing the fuel pump fuse or disconnecting the fuel line at the pump
4. Label all spark plug wires with cylinder numbers (1-4 passenger side front to rear, 5-8 driver side front to rear) before removal
5. Clean debris from around spark plug wells to prevent contamination from entering cylinders

Procedure

1. 1

   Remove all spark plug wires
   Grasp each spark plug wire boot firmly at the base near the spark plug and twist while pulling to remove. Do not pull on the wire itself. Remove all 8 wires and secure them away from the engine.
2. 2

   Remove all spark plugs
   Using the appropriate spark plug socket (5/8" for most 302 applications) and ratchet with extension, remove all 8 spark plugs. Inspect plugs for fouling, abnormal deposits, or damage which may indicate existing engine issues. Set plugs aside in order if you wish to analyze individual cylinder condition.
3. 3

   Disable ignition system
   Disconnect the coil wire from the center of the distributor cap and ground it to the engine block using a jumper wire or secure it away from any metal surfaces. This prevents voltage buildup and potential shock during cranking.
4. 4

   Prepare compression tester
   Select the correct threaded adapter for the 302 V8 spark plug holes and thread it hand-tight into the compression tester hose. Ensure the Schrader valve in the tester is functioning and the gauge reads zero before starting.
5. 5

   Test cylinder 1
   Thread the compression tester adapter into the number 1 cylinder spark plug hole (front passenger side) hand-tight. Have an assistant hold the throttle wide open and crank the engine for 5-6 compression strokes (about 3-4 seconds) or until the gauge needle stops climbing. Record the maximum reading. Release the pressure valve on the tester and remove.
6. 6

   Test remaining cylinders
   Repeat the testing procedure for cylinders 2 through 8 in sequence, recording each reading. Maintain consistent cranking duration (5-6 compression strokes) for each cylinder. Allow 30 seconds between tests to prevent starter overheating.
7. 7

   Analyze compression readings
   Compare all cylinder readings. Healthy 302 V8 compression should be 140-170 PSI depending on condition and altitude. No cylinder should vary more than 10-15% from the highest reading. Low readings in adjacent cylinders may indicate head gasket failure. Low reading in single cylinder suggests valve or ring issues.
8. 8

   Perform wet test if needed
   If any cylinder shows low compression (below 100 PSI or more than 20% below average), squirt approximately 1 tablespoon of engine oil into that cylinder through the spark plug hole. Retest compression. If reading increases significantly (30+ PSI), rings are worn. If reading stays similar, valves or head gasket are likely at fault.
9. 9

   Reinstall spark plugs
   Inspect spark plug threads and apply a small amount of anti-seize compound to the threads if desired (avoid getting any on electrodes). Thread each spark plug by hand into its respective cylinder hole to prevent cross-threading, then tighten with spark plug socket. Snug firmly but do not overtighten. Verified detail (The 1972 302 uses gasketed (flat seat) spark plugs with 5/8" (14mm) hex, not tapered-seat plugs. The AI best-guess of 15-20 ft-lbs is reasonable for gasketed plugs in cast iron heads, but the 'tapered seat' descriptor is incorrect for this engine.): Exact torque specification for spark plugs in 1972 Ford 302 V8 with gasketed (flat seat) spark plugs
10. 10

    Reconnect ignition and fuel systems
    Reconnect the coil wire to the distributor cap center terminal. Reinstall all spark plug wires to their correct cylinders following your labels (302 firing order 1-5-4-2-6-3-7-8). Restore fuel delivery by reconnecting the fuel line. Verified detail (No fuel pump fuse exists on a 1972 carbureted 302 with a mechanical fuel pump. This step references a nonexistent electric fuel system component.): No fuel system action is needed on this mechanical/carbureted engine beyond ensuring the throttle linkage returns to its normal closed position.

Reassembly

1. Double-check that all spark plug wires are firmly seated on plugs and routed away from hot exhaust manifolds
2. Verify coil wire is properly connected before starting engine
3. Clear any tools from engine bay

Verification

• Start engine and listen for smooth idle with no misfires
• Verify all spark plug wire connections are secure by checking for arcing or misfires
• Review compression test results: consistent readings across all cylinders indicate healthy engine; significant variations require further diagnosis

More procedures for this vehicle