Leak-Down Test

A leak-down test measures cylinder sealing by pressurizing each cylinder with compressed air and measuring the percentage of pressure loss, helping diagnose piston ring wear, valve seat damage, or head gasket failure on the 302 V8.

Warnings

Tools required

Parts

• Spark plug gaskets or new spark plugs (if existing plugs are fouled) × 8 — Autolite 25 or equivalent

Preparation

1. Allow engine to cool completely (minimum 2 hours after last operation) to ensure accurate readings and safe handling.
2. Park vehicle on level surface, engage parking brake, and place transmission in Park or 1st gear.
3. Ensure compressed air supply is regulated to 80-100 PSI with adequate CFM capacity for sustained pressure.
4. Verify leak-down tester is calibrated and functioning (both gauges should read identically with no restriction).
5. Remove air cleaner assembly to provide access to valve covers and improve working space.
6. Label all spark plug wires with cylinder number before removal to ensure correct reinstallation.

Procedure

1. 1

   Remove valve covers
   Remove the valve cover bolts from each valve cover on the 302 V8 (typically 6 bolts per cover). Lift valve covers off and set aside with gaskets. This allows visual confirmation of valve train position during TDC location and observation of rocker arm movement during the test.
2. 2

   Remove all spark plugs
   Disconnect all 8 spark plug wires, noting cylinder positions. Using a 5/8" spark plug socket, remove all 8 spark plugs. Inspect plugs for fouling, oil, or coolant contamination as preliminary indicators. Store plugs in order or label them by cylinder for reference.
3. 3

   Rotate engine to cylinder 1 TDC compression
   Rotate the crankshaft clockwise (viewed from front) using a socket on the crankshaft pulley bolt until cylinder 1 is at top dead center on the compression stroke. Verify TDC compression by confirming both intake and exhaust valves are closed (rocker arms loose) and the timing mark on the harmonic balancer aligns with the 0-degree mark on the timing cover. Ford 302 firing order is 1-5-4-2-6-3-7-8.
4. 4

   Install leak-down tester in cylinder 1
   Thread the leak-down tester adapter fitting hand-tight into cylinder 1 spark plug hole. Ensure the fitting is seated properly. Connect the tester hose from the compressed air supply to the leak-down tester input, and connect the cylinder hose to the adapter. Do not apply air pressure yet.
5. 5

   Perform cylinder 1 leak-down test
   Slowly apply compressed air pressure to the leak-down tester while holding the crankshaft in position. The input gauge should read 80-100 PSI (your regulated supply pressure). Read the output gauge, which shows cylinder pressure. Calculate leak-down percentage: ((input PSI - output PSI) / input PSI) × 100. Record the percentage for cylinder 1. Acceptable leak-down is 5-10%, marginal is 10-20%, and above 20% indicates significant wear or damage. While pressurized, listen at the throttle body (intake valve leak), exhaust tailpipe (exhaust valve leak), oil filler cap (ring leak), and radiator (head gasket leak) to identify leak source.
6. 6

   Test remaining cylinders in firing order
   Release air pressure and remove tester from cylinder 1. Rotate the crankshaft to bring the next cylinder in the firing order (cylinder 5) to top dead center on the compression stroke. On a V8 a firing event occurs every 90 degrees of crankshaft rotation, but always verify TDC compression by confirming both valves are closed rather than relying on the rotation angle alone. Repeat the leak-down test procedure, recording results and listening for leak sources. Continue through the firing order: cylinders 1-5-4-2-6-3-7-8, verifying TDC compression before each test. This covers two full crankshaft rotations through all 8 cylinders (720 degrees total for the 4-stroke cycle, with a firing event every 90 degrees).
7. 7

   Interpret and record results
   Compare all cylinder leak-down percentages. Variation between cylinders should be less than 5%. Document any cylinders exceeding 20% leak-down or showing more than 10% variance from others. Note the primary leak source for problem cylinders: carburetor/throttle (intake valve), tailpipe (exhaust valve), crankcase/oil filler (rings or cylinder wall), or radiator/coolant (head gasket). Hissing heard between cylinders indicates head gasket failure between adjacent combustion chambers.
8. 8

   Reinstall spark plugs
   Apply anti-seize compound sparingly to spark plug threads if desired (avoid getting compound on electrodes). Hand-thread each spark plug into its correct cylinder location to prevent cross-threading. Tighten spark plugs to specification. Without a documented torque specification for this application, tighten to 15-20 ft-lbs for these 14mm gasketed (flat seat) plugs using a torque wrench, or if a torque wrench is unavailable, seat the plug finger-tight then turn approximately 1/2 turn for a new gasket (1/16 turn if reusing the same plug/gasket). Thread chaser or tap (14mm x 1.25).

Reassembly

1. Reconnect spark plug wires to correct cylinders following labels applied during disassembly. Verify wire routing matches original configuration with wires seated fully onto plugs.
2. Install valve covers with gaskets in correct orientation. Tighten valve cover bolts evenly in a cross pattern to prevent gasket distortion and oil leaks.
3. Reinstall air cleaner assembly and verify all vacuum lines and breather hoses are reconnected.
4. If battery was disconnected, reconnect negative terminal.

Verification

• Start engine and verify smooth idle with no misfires or unusual noises. All cylinders should contribute evenly to idle quality.
• Check for oil leaks at valve covers and verify no air or vacuum leaks from spark plug holes.
• Review recorded leak-down percentages: all cylinders below 10% indicates excellent compression sealing; 10-20% may be acceptable for a vintage engine with moderate wear; above 20% or significant cylinder-to-cylinder variation warrants further diagnosis or repair.
• Compare leak-down results with any previous compression test data if available. Leak-down test provides more specific diagnosis than compression testing alone.

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