Compression Test

Performs a compression test on all 12 cylinders of the 6.0L V12 engine to diagnose engine mechanical condition, including removal of intake plenum and ignition coils for spark plug access.

Warnings

Tools required

Parts

• Spark plugs (if fouled or damaged during removal) × 12 — NGK BKR7EIX or OEM equivalent

Preparation

1. Park vehicle on level surface and engage parking brake.
2. Allow engine to cool completely (at least 2 hours after last operation).
3. Disconnect negative battery terminal and install battery memory saver if available.
4. Remove engine cover by lifting upward from rear clips and pulling forward to disengage front locating tabs.
5. Photograph or label all vacuum lines and electrical connectors before disconnection for reference during reassembly.
6. Have compression test data sheet ready with cylinder numbering diagram (Bank A: cylinders 1-6 passenger side, Bank B: cylinders 7-12 driver side, numbered front to rear).

Procedure

1. 1

   Remove intake air system
   Disconnect mass airflow sensor electrical connector. Release air intake duct clamps at throttle body and airbox using flathead screwdriver or 7mm nut driver. Remove intake duct. Disconnect breather hoses from valve covers. Release airbox mounting clips and remove airbox assembly from engine bay.
   Torque spec

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

   Disconnect fuel system and ignition power
   Locate the fuel pump fuse or relay (consult the vehicle-specific fuse layout; on the DB9 the fuse boxes are not in the engine bay — consult the vehicle-specific fuse layout for exact locations (commonly in the luggage/boot area and cabin; verify against Aston Martin documentation)). Remove the fuel pump fuse. Start engine and allow it to run until it stalls to depressurize fuel system. Attempt to start again to confirm fuel pressure is relieved. Disable the ignition system to prevent coils firing during cranking (disconnect the ignition coil harness connector(s) per the vehicle-specific wiring diagram). Verify ignition power to the coils is removed before cranking. Note: Disable the ignition system to prevent spark during cranking (disconnect the coil harness connectors per the vehicle wiring diagram, or disable ignition via the appropriate fuse/relay — verify the correct method against Aston Martin service documentation). Note: Locate the fuel pump fuse or relay (consult the vehicle fuse box diagram for exact location and designation — the DB9 fuse boxes are located in the luggage compartment and cabin). Remove the fuel pump fuse. Note: Locate the fuel pump fuse or relay (consult the vehicle-specific fuse diagram — on the DB9 the fuse panels are located in the boot and passenger-side footwell, not the engine bay). Remove the fuel pump fuse.
3. 3

   Remove intake plenum assembly
   Disconnect throttle body electrical connector and coolant hoses (mark positions). Remove the intake plenum mounting bolts securing the plenum to the cylinder heads (exact count and fastener type must be confirmed against factory documentation for this engine; inspect perimeter and center mounting points). Carefully lift plenum assembly upward, disconnecting any remaining vacuum lines or PCV connections. Set plenum aside on protective surface to avoid scratching. Note: Remove the intake plenum mounting bolts securing the plenum to the cylinder heads (verify exact fastener count and drive type against Aston Martin service documentation; inspect perimeter and center mounting points). Note: Remove the intake plenum mounting bolts (verify exact count and Torx size against OEM documentation for this engine; inspect perimeter and center mounting points).
4. 4

   Access ignition coils on Bank A (passenger side)
   Identify all 6 ignition coils on Bank A (cylinders 1-6, front to rear). Each coil has an individual electrical connector; press release tab and disconnect each connector. Remove the single bolt securing each coil (verify fastener size against factory documentation for this engine). Grasp coil firmly and pull straight up with twisting motion to break rubber seal. If coil resists, use ignition coil puller tool to avoid damage.
5. 5

   Access ignition coils on Bank B (driver side)
   Repeat coil removal process for Bank B (cylinders 7-12). Disconnect each coil electrical connector by pressing release tab. Remove the single mounting bolt from each coil (verify fastener size and drive type against factory documentation for this engine). Extract coils carefully using straight upward pulling motion with slight twist. Set all coils aside in order to maintain cylinder identification.
6. 6

   Remove spark plugs from all cylinders
   Use compressed air to blow debris from around each spark plug well to prevent contamination entering cylinders. Using the correct thin-wall spark plug socket (verify plug hex size against the plug/factory specification for this engine), carefully remove all 12 spark plugs. Maintain organization by cylinder number. Inspect each plug for fouling, oil contamination, or abnormal wear patterns as diagnostic clues. Note any plugs requiring replacement. Note: Using a thin-wall spark plug socket of the correct size for the OEM plug (verify hex size against Aston Martin service documentation), carefully remove all 12 spark plugs. Note: Using a thin-wall spark plug socket of the correct hex size for this engine (verify against OEM spec — commonly 16mm/5/8" on this V12), carefully remove all 12 spark plugs.
7. 7

   Prepare compression tester and throttle
   Install the correct thread adapter into the compression tester (verify spark plug thread size against Aston Martin service documentation) if not integral. Verify tester gauge reads zero. Ensure throttle plate is in wide-open position (use assistant to hold accelerator pedal to floor, or prop throttle body butterfly open manually if accessible). This ensures unrestricted airflow during cranking for accurate readings. Note: Install the thread adapter matching the engine's spark plug thread size into the compression tester if not integral (verify plug thread size before selecting the adapter). Note: Install the correct thread adapter into the compression tester (verify spark plug thread size against OEM spec for this engine; commonly M14x1.25) if not integral.
8. 8

   Test compression on Bank A cylinders 1-6
   Thread compression tester firmly into cylinder 1 spark plug hole (hand-tight plus 1/4 turn). Have assistant crank engine using ignition key, or use remote starter switch at starter solenoid. Crank for 5-6 compression strokes (engine will turn approximately 3-4 seconds). Record peak reading. Release tester pressure valve and repeat for cylinders 2 through 6. Record all readings with cylinder numbers.
9. 9

   Test compression on Bank B cylinders 7-12
   Move to Bank B and repeat compression test procedure for cylinders 7 through 12. Thread tester into each cylinder, crank engine for 5-6 strokes, and record peak compression reading. Note any cylinders with significantly low readings (more than 10% below the highest reading, or below the minimum compression figure specified by Aston Martin for this engine). Note: Note any cylinders with significantly low readings (more than 10% variance from the highest reading, or below the minimum compression value specified in Aston Martin documentation). Note: Note any cylinders with significantly low readings (more than 10% below the highest reading, or below the manufacturer's minimum compression specification for this engine).
10. 10

    Perform wet test on low-compression cylinders (if needed)
    If any cylinder shows low compression (more than 10% below the highest reading, or below the manufacturer's minimum spec), perform wet compression test to diagnose ring vs. valve issues. Squirt approximately 1 tablespoon (15ml) of clean engine oil into the suspect cylinder through spark plug hole. Reinstall compression tester and retest. If compression increases significantly (30+ psi), rings are likely worn. If compression remains low, suspect valve sealing or head gasket issues. Note: If any cylinder shows low compression (below the minimum compression value specified by Aston Martin, or more than 10% variance from the highest cylinder), perform wet compression test
11. 11

    Clean and inspect spark plug threads
    Use compressed air to thoroughly clean each spark plug well. Inspect threads in each cylinder head for damage or carbon buildup. If installing new plugs, follow the plug manufacturer's guidance regarding anti-seize (many OEM plugs are pre-coated and should be installed dry); do not apply compound to the electrodes. If reusing plugs and they show normal wear, gap check is recommended before reinstallation.
12. 12

    Reinstall spark plugs
    Install spark plugs into their respective cylinders, threading by hand first to avoid cross-threading. Torque plugs to the factory specification (verify exact spark plug torque value against Aston Martin service documentation before tightening to avoid damaging the aluminum heads; do not rely on a generic figure). Use torque wrench to ensure proper seating without overtorque risk to aluminum heads. Note: If installing new plugs, install dry unless the plug or vehicle manufacturer specifically calls for anti-seize; most modern plated plugs should NOT have anti-seize applied as it can cause overtorque and thread damage in aluminum heads. Note: Torque plugs to the factory specification for this engine (exact value must be confirmed from Aston Martin documentation; do not rely on a generic figure). Note: Install plugs dry unless the plug/OEM documentation specifies anti-seize; most modern plated plugs should NOT receive anti-seize as it alters torque and can damage threads. Note: Torque plugs to the factory specification for this engine (verify against OEM data — for 14mm plugs in aluminum heads this is often around 15 Nm / 11 lb-ft; do not overtorque).
13. 13

    Reinstall ignition coils
    Apply small amount of dielectric grease to inside of coil boot. Press each ignition coil firmly into its spark plug well until fully seated (you should feel/hear it snap onto plug terminal). Install coil mounting bolt and tighten to the factory specification for this engine (verify torque value against Aston Martin documentation). Reconnect electrical connector to each coil, ensuring positive lock engagement. Verify all 12 coils are secured and connected. Note: Install the coil mounting bolt and tighten to the factory specification (verify exact coil bolt torque value against Aston Martin service documentation). Note: Install coil mounting bolt and tighten to the OEM-specified torque (verify against factory data for this engine).
14. 14

    Reinstall intake plenum
    Inspect plenum gaskets for damage; replace if torn or compressed. Position plenum carefully over cylinder heads, reconnecting vacuum lines and PCV connections as it lowers into place. Install all plenum mounting bolts finger-tight first, then torque to the factory specification in the sequence given in Aston Martin service documentation to ensure even gasket compression. Reconnect throttle body electrical connector and coolant hoses. Note: Install all plenum mounting bolts finger-tight first, then torque in the sequence and to the value specified by Aston Martin for this engine (confirm both against factory documentation before final tightening). Note: Install all plenum mounting Torx bolts finger-tight first, then torque in the OEM-specified sequence and value for this engine (verify against factory data).
15. 15

    Reinstall air intake system and finalize
    Position airbox and secure mounting clips. Reconnect breather hoses to valve covers. Install intake duct and tighten clamps at throttle body and airbox. Reconnect mass airflow sensor connector. Reinstall fuel pump fuse. Reconnect main ignition coil power connectors on both banks. Reconnect battery negative terminal. Reinstall engine cover by engaging front tabs first, then pressing down at rear clips until secured.

Reassembly

1. Ensure all electrical connectors are fully seated with positive locks engaged; incomplete connections will trigger check engine light.
2. Verify no tools or rags were left in engine bay, particularly in intake plenum area or valley between cylinder banks.
3. Double-check that fuel pump fuse was reinstalled and all vacuum lines are reconnected per reference photos.
4. After first start, inspect for any fuel, coolant, or vacuum leaks around reassembled components.

Verification

• Start engine and verify smooth idle with no misfires or rough running.
• Check that all compression readings are within specification: 170-190 psi with no more than 10% variance between highest and lowest cylinders.
• Confirm no check engine light or diagnostic trouble codes related to ignition or fuel system.
• Perform short test drive to verify normal power delivery and smooth acceleration through rev range.
• If compression was low on any cylinder, interpret results: wet test showed improvement = ring wear; no improvement = valve or head gasket issue; adjacent cylinders low = head gasket suspect between those cylinders.

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