Compression Test

Perform a compression test on all eight cylinders of the 390ci V8 engine to evaluate engine mechanical condition and identify potential issues with valves, rings, or gaskets.

Warnings

Tools required

Parts

• Spark plug anti-seize compound × 1 — Use OEM specification
• Dielectric grease for spark plug boots × 1 — Use OEM specification

Preparation

1. Ensure engine is at normal operating temperature, then shut off and allow to cool for 15-20 minutes for a warm compression test, or test completely cold
2. Verify battery is fully charged and connections are clean and tight
3. Locate and identify all eight spark plugs on the 390ci V8 (four per cylinder bank)
4. Prepare a chart to record compression readings for cylinders 1-8 in firing order: 1-8-4-3-6-5-7-2
5. Position vehicle on level ground with transmission in Park and parking brake applied

Procedure

1. 1

   Disable the ignition system
   Disconnect the primary wire from the negative (-) terminal of the ignition coil to prevent spark generation during cranking. On the 1970 AMC 390, the coil is typically mounted on the intake manifold or inner fender. Mark or note the wire position for reinstallation.
2. 2

   Remove air cleaner assembly
   Remove the wing nut securing the air cleaner lid, then lift off the entire air cleaner assembly from the carburetor. Set aside in a clean area. This provides access to the carburetor and prevents debris from entering the intake.
3. 3

   Block the throttle open
   Manually hold or secure the carburetor throttle plates in the wide-open position. This can be done by having an assistant press the accelerator pedal to the floor during cranking, or by using a throttle linkage block. Wide-open throttle ensures maximum airflow during compression testing for accurate readings.
4. 4

   Remove all spark plug wires
   Carefully remove all eight spark plug wires from the spark plugs by pulling only on the boot, never the wire itself. Twist the boot slightly while pulling to break the seal. Label wires 1-8 if not already marked to ensure correct reinstallation. The firing order is 1-8-4-3-6-5-7-2 with cylinder 1 at the front of the driver side bank.
5. 5

   Remove all spark plugs
   Using the appropriate spark plug socket (typically 5/8" for this application), remove all eight spark plugs. Keep them in order if you plan to read their condition. Inspect the plug wells for debris or oil and clean if necessary. Removing all plugs reduces cranking resistance and ensures consistent engine cranking speed for all cylinders.
6. 6

   Test cylinder 1 compression
   Install the compression gauge into cylinder #1 spark plug hole (front cylinder on driver side). Ensure a tight seal. Using the remote starter switch or having an assistant crank the engine, crank for 5-6 compression strokes (typically 3-4 seconds) while monitoring the gauge. Record the maximum reading achieved. Note the number of strokes to reach maximum pressure.
7. 7

   Test remaining cylinders in sequence
   Repeat the compression test procedure for cylinders 2 through 8. Remove the gauge, install in the next cylinder, and crank the engine for the same duration (5-6 strokes). Record each reading. Test cylinders in a logical pattern: driver side bank (1-3-5-7 front to rear), then passenger side (2-4-6-8 front to rear). Healthy 390ci engines typically show 130-160 psi depending on condition and elevation.
8. 8

   Analyze compression readings
   Compare all eight cylinder readings. Variation between cylinders should not exceed 10-15% of the highest reading. A low reading in one cylinder may indicate valve or ring problems. Low readings in adjacent cylinders suggest head gasket failure. Uniformly low readings across all cylinders indicate general wear. For suspect cylinders, perform a wet test by adding approximately 1 tablespoon of engine oil through the spark plug hole and retesting.
9. 9

   Interpret wet test results if performed
   If a wet test was performed on low-compression cylinders, compare wet versus dry readings. A significant increase (30+ psi) indicates worn piston rings or cylinder wall wear, as the oil temporarily seals the rings. Little to no increase suggests valve sealing issues, such as burnt valves, worn valve seats, or incorrect valve adjustment.
10. 10

    Clean and reinstall spark plugs
    Inspect spark plug threads and cylinder head threads for damage. Apply a small amount of anti-seize compound to the spark plug threads (avoid getting compound on electrodes). Hand-thread each spark plug into its original cylinder location, then tighten to proper torque. Without a torque specification, tighten 5/8" reach plugs 1/4 turn past hand-tight if new, or 1/16 to 1/8 turn if reinstalling used plugs with pre-compressed gaskets.
11. 11

    Reinstall spark plug wires
    Apply a small amount of dielectric grease inside each spark plug boot. Reinstall each spark plug wire to its corresponding cylinder according to your labels or the firing order 1-8-4-3-6-5-7-2. Ensure each boot seats fully onto the spark plug terminal with a positive click. Route wires through original separators and looms to prevent cross-firing.
12. 12

    Reconnect ignition system
    Reconnect the primary wire to the negative terminal of the ignition coil. Ensure the connection is clean and tight. Verify all electrical connections disturbed during the procedure are properly restored.
13. 13

    Reinstall air cleaner assembly
    Position the air cleaner assembly back onto the carburetor, ensuring the gasket seats properly. Secure with the wing nut, tightening hand-tight only.

Reassembly

1. Verify all spark plug wires are routed correctly and fully seated to prevent misfires
2. Double-check that the ignition coil primary wire has been reconnected
3. Confirm all tools and materials have been removed from the engine compartment
4. Start the engine and verify smooth operation with no misfires or unusual sounds

Verification

• Engine should start normally and idle smoothly without misfires
• All compression readings should be documented with cylinder number, psi reading, and any observations
• Normal compression for a healthy 390ci V8 is typically 130-160 psi with no more than 10-15% variation between cylinders
• Any cylinder reading below 100 psi or showing more than 20% variation from others indicates mechanical issues requiring further diagnosis
• Wet test results, if performed, should be documented to distinguish between ring and valve problems

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