Leak-Down Test

Perform a leak-down test on the 6.0L V12 engine to measure cylinder sealing integrity and diagnose compression loss through valves, rings, or head gasket.

Warnings

Tools required

Parts

• Spark plug set (if condition warrants replacement) × 12 — Use OEM specification
• Ignition coil seals/boots (if damaged during removal) × 12 — Use OEM specification

Preparation

1. Park vehicle on level ground and engage parking brake.
2. Warm engine to full operating temperature (coolant at 80-90°C) then shut off and allow 15-20 minutes cooling time for safe handling.
3. Open hood and secure with prop rod.
4. Disconnect battery negative terminal and secure cable away from terminal.
5. Remove engine cover by releasing clips or fasteners (typically push clips or Torx fasteners at perimeter).
6. Photograph ignition coil layout and connector positions for reference during reassembly.

Procedure

1. 1

   Remove intake air system components
   Disconnect the intake air duct from the throttle body. Release the air box clips and remove the air filter housing to gain access to the rear bank of cylinders. The DB9 V12 has individual throttle bodies; you may need to remove decorative covers or acoustic panels secured with Torx fasteners to access all ignition coils.
   Torque spec

   Air Box Clips5 Nm (4 lb-ft)
2. 2

   Remove ignition coils from all cylinders
   Disconnect the electrical connector from each of the 12 ignition coils by pressing the release tab. Remove the coil hold-down fasteners if present (verify fastener type and size against Aston Martin service documentation; some coils may be press-fit rather than bolted). Pull each coil straight up from the spark plug well using steady force; use a coil puller if resistance is encountered. Inspect coil boots for damage and place coils in numbered order matching cylinder position (front bank 1-6, rear bank 7-12). Note: Remove the coil hold-down fasteners (verify exact Torx size and fastener type against factory Aston Martin documentation for this engine). Note: Remove the coil hold-down fasteners (verify exact Torx size against OEM documentation before removal). Note: Fastener type, size, and torque to be verified against Aston Martin factory service documentation (coils may be press-fit rather than bolted on this engine)
3. 3

   Remove all spark plugs
   Using a thin-wall spark plug socket of the correct size (verify exact spark plug socket hex size against factory Aston Martin documentation) with extension, remove all 12 spark plugs. Keep plugs organized by cylinder number for inspection. Examine each plug for oil fouling, carbon deposits, or electrode wear that may indicate cylinder-specific issues. Cover open spark plug wells with clean shop towels to prevent debris entry. Note: Using a thin-wall spark plug socket of the correct size (verify 14mm vs 16mm against the OEM spark plug specification) with an extension, remove all 12 spark plugs. Note: Using the correct thin-wall spark plug socket (verify 14mm vs 16mm hex against OEM specification) with extension, remove all 12 spark plugs.
4. 4

   Disable fuel and ignition systems
   With battery already disconnected, remove the fuel pump fuse or relay to prevent fuel delivery during testing. This step is precautionary as the battery disconnect already interrupts the ignition system. Verify the throttle is closed and no debris is present in intake tract.
5. 5

   Rotate cylinder 1 to TDC compression stroke
   Remove the shop towel from cylinder 1 spark plug well. Using a socket of the correct size on the crankshaft pulley bolt (verify exact size against factory Aston Martin documentation), rotate the engine clockwise while monitoring cylinder 1. Insert a piston stop tool or soft copper bar into the spark plug hole to feel when the piston reaches TDC. Verify TDC by confirming both intake and exhaust valves are closed (check with light finger pressure on valve covers if rockers are accessible, or use timing marks on crankshaft pulley if visible). The compression stroke is when both valves are closed at top position. Note: Using a socket of the correct size on the crankshaft pulley bolt (verify size against Aston Martin service documentation), rotate the engine clockwise while monitoring cylinder 1. Note: Using the correct socket on the crankshaft pulley bolt (verify size against OEM documentation), rotate the engine clockwise while monitoring cylinder 1. Note: Breaker bar with crankshaft pulley socket (verify exact size against Aston Martin service documentation)
6. 6

   Perform leak-down test on cylinder 1
   Set the leak-down tester regulator to 80-100 psi input pressure. Thread the leak-down tester adapter into cylinder 1 spark plug hole. Apply air pressure gradually while monitoring both gauges. Record the percentage of leakage shown on the tester (acceptable range is typically 5-10% or less; 10-20% indicates wear; above 20% indicates significant problems). While air is applied, listen at the throttle body (intake valve leakage), oil filler cap (ring leakage), and exhaust tailpipe (exhaust valve leakage) to identify leak source. Listen at the radiator cap for head gasket leakage into coolant jacket. Note all findings for cylinder 1.
7. 7

   Test remaining cylinders in sequence
   Remove the leak-down tester from cylinder 1. Rotate the crankshaft clockwise to bring the next cylinder in the firing order to its TDC compression stroke (confirm the exact firing order and cylinder numbering against Aston Martin service documentation; a V12 has firing events every 60 degrees of crank rotation, but verify the precise rotation required to reach each cylinder's compression TDC against factory documentation rather than assuming a fixed interval). Verify each cylinder with the piston stop method before testing. Use the piston stop method to confirm TDC with both valves closed. Repeat the leak-down test procedure for cylinder 2, recording results. Continue this process for all 12 cylinders in sequence: position piston at TDC compression, apply air pressure, measure leakage percentage, identify leak sources, record data. This systematic approach ensures consistent results across the engine. Note: Remove the leak-down tester from cylinder 1. To test cylinders efficiently, rotate the crankshaft to bring the next cylinder in the firing order to TDC compression (each firing event is 60 degrees of crankshaft rotation apart on a V12). Confirm the exact firing order and cylinder numbering against factory Aston Martin documentation before sequencing, then use the piston stop method to confirm TDC with both valves closed. Repeat the leak-down test procedure for each cylinder, recording results until all 12 cylinders have been tested. Note: Rotate the crankshaft clockwise approximately 60 degrees to position the next cylinder in the firing order at TDC compression stroke (firing intervals on a V12 occur every 60 degrees of crank rotation). Do NOT rely on an assumed firing order — verify the exact Aston Martin DB9 6.0L V12 firing order and cylinder numbering against OEM documentation before sequencing cylinders. Note: V12 firing order must be confirmed from OEM service documentation (do not assume a sequence)
8. 8

   Analyze leak-down test results
   Review recorded data for all 12 cylinders. Compare leakage percentages: consistent readings across cylinders indicate normal uniform wear; one or two cylinders with high leakage indicate localized problems (burned valves, broken rings, or cylinder scoring); adjacent cylinders with high leakage may indicate head gasket failure between those cylinders. Cross-reference leak sources: intake noise suggests intake valve issues, exhaust noise suggests exhaust valve problems, oil filler noise indicates ring or cylinder wall wear, coolant bubbling indicates head gasket or crack issues. Document findings for repair planning.
9. 9

   Inspect spark plug wells and prepare for reassembly
   Remove all shop towels from spark plug wells. Inspect each well for oil accumulation (indicates valve cover gasket or seal leakage). Clean any debris or oil from wells using compressed air and clean towels. Verify all spark plug threads in cylinder head are clean and undamaged.
10. 10

    Reinstall spark plugs
    Install all 12 spark plugs by hand first to avoid cross-threading, then torque to the manufacturer specification (verify exact value against Aston Martin service documentation). If original plugs show excessive wear or fouling, replace with new plugs gapped to specification. Install plugs in the same cylinder positions if reusing, or install new complete set if replacing. Note: Install all 12 spark plugs by hand first to avoid cross-threading, then torque to manufacturer specification (verify exact torque value against factory Aston Martin documentation). Note: then torque to manufacturer specification (verify exact spark plug torque against OEM documentation before final tightening) Note: Verify exact spark plug torque against Aston Martin factory service documentation
11. 11

    Reinstall ignition coils
    Apply a thin coat of dielectric grease to spark plug terminals and inside coil boots if new boots are used. Install each ignition coil into its correct cylinder position by pressing firmly until fully seated. Install the coil hold-down fasteners if present and torque to the manufacturer specification (verify exact value against Aston Martin service documentation). Reconnect all 12 coil electrical connectors, ensuring each locks with an audible click. Note: Install the coil hold-down fasteners and torque to specification (verify exact torque value against factory Aston Martin documentation). Note: Install the coil hold-down fasteners and torque to specification (verify exact coil hold-down torque against OEM documentation). Note: Verify exact coil hold-down torque against Aston Martin factory service documentation (if hold-down fasteners are present)
12. 12

    Reinstall intake air system and engine covers
    Reinstall any acoustic panels or decorative covers removed for coil access. Reinstall the air filter housing and secure with clips. Reconnect the intake air duct to the throttle body. Reinstall the main engine cover and secure all fasteners or clips.
13. 13

    Reconnect battery and restore fuel system
    Reconnect the battery negative terminal. Reinstall the fuel pump fuse or relay. Turn the ignition key to ON position (without starting) for 5 seconds, then OFF, then ON again to prime the fuel system and check for warning lights. The ECU may need to relearn idle parameters.

Reassembly

1. Torque all spark plugs to specification to prevent blow-by or thread damage.
2. Ensure all 12 ignition coil connectors are fully seated and locked to prevent misfires.
3. Verify all intake system clamps and connections are secure to prevent air leaks affecting idle quality.
4. Check that no tools or shop towels were left in the engine bay before closing hood.

Verification

• Start the engine and listen for smooth idle with no misfires or abnormal noises.
• Check for fault codes using a diagnostic scanner; clear any codes set during testing with ignition disabled.
• Verify no air leaks at intake connections or vacuum lines.
• Review leak-down test data: acceptable engines show 5-10% leakage or less per cylinder with consistency across all cylinders; values above 15% warrant further diagnosis or repair.
• If significant leakage was found, correlate findings with symptoms (low compression, oil consumption, coolant loss) to plan appropriate repairs.

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