Coolant Bypass Pipe

Replace the coolant bypass pipe on a 2009 Aston Martin DB9 6.0L V12, requiring partial cooling system disassembly, engine cover removal, and access from both sides of the V12 engine bay.

Warnings

Tools required

Parts

• Coolant bypass pipe × 1 — Use OEM specification
• Bypass pipe O-rings/gaskets × 1 — Usually included with pipe
• Hose clamps (if corroded) × 4 — Metric worm-drive type

Fluids

• OAT 50/50 coolant — 11 qt

Preparation

1. Ensure engine is completely cold to the touch
2. Park vehicle on level ground and engage parking brake
3. Place coolant drain pan beneath engine
4. Remove engine cover by releasing quick-release fasteners at perimeter (typically 6 points around cover edge)
5. Disconnect negative battery terminal to prevent accidental cooling fan activation
6. Remove plastic engine undertray if equipped (Torx fasteners, typically T30)

Procedure

1. 1

   Drain Cooling System
   Locate the lower radiator drain point. On the DB9 this may be a drain plug/petcock or may require disconnecting the lower radiator hose to drain — verify the correct drain method against the factory workshop manual before proceeding. Place drain pan beneath radiator and remove drain plug carefully. Allow coolant to drain completely into pan. If equipped with a petcock valve instead of plug, open valve fully and allow to drain. Once draining slows, carefully remove radiator cap to allow complete drainage. Note: Locate the coolant drain point on the lower radiator (driver's side). Place drain pan beneath radiator and open the drain carefully. Allow coolant to drain completely into pan. If equipped with a petcock valve, open valve fully and allow to drain. Once draining slows, remove the expansion/header tank cap to relieve vacuum and allow complete drainage. Note: Locate the lower radiator hose or coolant drain point at the front of the vehicle. Place a drain pan beneath and loosen the lower radiator hose clamp (or open the drain point if equipped) to allow coolant to drain completely into the pan. Remove the expansion/header tank cap (engine must be cold) to vent the system and allow complete drainage. Note the DB9 does not use a conventional radiator cap.
   Torque spec

   Coolant Drain Plug15 Nm (11 lb-ft)
2. 2

   Remove Air Intake Components
   Disconnect the air intake tubes from the throttle body/bodies (verify throttle body count and configuration for this engine against the factory manual). Release clamps at each throttle body connection and at airbox outlets. Remove the airbox assembly by releasing mounting fasteners (typically Torx T30). Disconnect any breather hoses or vacuum lines attached to intake components. Label connections for reinstallation. Set components aside to provide clear access to engine valley. Note: Disconnect the air intake tubes from the throttle body/intake inlet(s). (Verify throttle body configuration against factory documentation—the V12 intake arrangement may differ from the two-throttle-body assumption.)
3. 3

   Remove Upper Engine Covers and Plenum
   Remove the upper intake plenum/manifold covers on both cylinder banks. These are typically secured with multiple Torx fasteners (T30 or T40) around the perimeter. Disconnect any vacuum lines, breather hoses, or electrical connectors attached to the plenum. Work carefully as these covers are typically cast aluminum or composite. The V12 has covers on both banks that must be removed to access the valley area.
4. 4

   Disconnect Coolant Hoses at Valley Access Points
   Locate the coolant hoses connected to the bypass pipe in the engine valley. Use hose clamp pliers to release spring clamps or a screwdriver to loosen worm-drive clamps. The bypass pipe typically has 2-2-3 hose connections (verify exact count against factory service data). Gently twist hoses to break the seal before pulling off the pipe fittings. Have shop towels ready to catch residual coolant. Note the exact routing and orientation of each hose for reinstallation.
   Torque spec

   Hose Clamps3 Nm (2 lb-ft)
5. 5

   Remove Thermostat Housing (If Required for Access)
   Depending on bypass pipe routing, you may need to remove or loosen the thermostat housing for clearance. The housing is typically secured with several bolts (verify exact count and head type against factory service data before disassembly). Remove bolts and carefully separate housing. The thermostat may remain with the housing or in the block—note its orientation for reinstallation. Keep the gasket or O-ring with the housing. Note: The housing is secured with multiple bolts (verify exact count and fastener type against the factory workshop manual before selecting tools). Note: The housing is typically secured with 2-3 bolts (fastener type/size must be verified against factory documentation—do not assume E-Torx).
   Torque spec

   Thermostat Housing Bolt15 Nm (11 lb-ft)
6. 6

   Remove Bypass Pipe Mounting Fasteners
   Locate the bypass pipe mounting bolts in the engine valley. The pipe is secured with mounting bolts to the block or water pump housing (verify exact count and head type against factory service data before disassembly). Use an extension bar and swivel socket for tight access angles. Remove all fasteners and keep track of any brackets or spacers that may be present. Check for any additional clips or retaining tabs securing the pipe. Note: The pipe is secured with mounting bolts to the block or water pump housing — verify exact count and fastener type against the factory workshop manual before beginning. Note: The pipe is secured with mounting bolts to the block or water pump housing; the exact count and fastener drive type must be verified against factory documentation before purchasing tools. Note: Fastener count and drive type unverified — confirm exact bypass pipe mounting bolt count and head type against Aston Martin factory service data before selecting tools.
7. 7

   Extract Old Bypass Pipe
   Carefully work the bypass pipe out of the engine valley. The pipe may be wedged between components and require rotation or angling to extract. Do not force—if resistance is encountered, verify all fasteners and hose connections have been removed. Inspect the pipe for cracks, corrosion, or damage to confirm replacement is necessary. Drain any residual coolant from the pipe into your drain pan.
8. 8

   Clean Mounting Surfaces and Inspect Sealing Areas
   Thoroughly clean all mounting surfaces where the bypass pipe seals to the engine block, water pump, or thermostat housing. Remove old gasket material, corrosion, or debris using a plastic scraper (never metal on aluminum surfaces). Wipe surfaces clean with lint-free shop towels. Inspect mating surfaces for cracks, warping, or damage. Check that coolant passages are clear and not obstructed.
9. 9

   Prepare New Bypass Pipe
   Unpack the new bypass pipe and verify it matches the original in configuration and hose connection points. Install new O-rings or gaskets on the pipe fittings—lightly lubricate sealing surfaces with clean coolant to ease installation. Verify all sealing surfaces on the new pipe are clean and undamaged. Compare new and old parts side-by-side to confirm correct replacement part.
10. 10

    Install New Bypass Pipe
    Carefully position the new bypass pipe into the engine valley, routing it along the same path as the original. Align mounting holes with block or housing bosses. Start all mounting bolts by hand to ensure proper thread engagement. Once all bolts are started, tighten them progressively and evenly to avoid distorting the pipe or sealing surfaces. Use a torque wrench to tighten the mounting bolts to the manufacturer-specified torque (consult the factory workshop manual for the exact value — do not assume). Note: Use a torque wrench to achieve the factory-specified torque for the mounting bolts (verify specification against Aston Martin service data before final tightening). Note: Use a torque wrench to tighten the mounting bolts to the factory-specified torque (value must be obtained from Aston Martin service documentation—do not rely on an assumed figure). Note: Torque value not verified — obtain the exact bypass pipe bolt torque from Aston Martin factory service documentation before final tightening.
    Torque spec

    Thermostat Housing Bolt15 Nm (11 lb-ft)
11. 11

    Reconnect Coolant Hoses
    Reconnect all coolant hoses to the bypass pipe fittings in the order noted during removal. Ensure hoses are fully seated on each fitting—you should feel them bottom out against the pipe shoulder. Position hose clamps 1/4 inch back from the fitting end and tighten to specification. Verify clamps are not over-tightened, which can damage hoses. Double-check that no hoses are kinked or stressed.
    Torque spec

    Hose Clamps3 Nm (2 lb-ft)
12. 12

    Reinstall Thermostat Housing (If Removed)
    If the thermostat housing was removed, reinstall the thermostat in its original orientation (spring side typically toward engine). Position a new gasket or O-ring on the housing. Align housing with block and start all bolts by hand. Tighten bolts in a criss-cross pattern to specification to ensure even gasket compression.
    Torque spec

    Thermostat Housing Bolt15 Nm (11 lb-ft)
13. 13

    Reinstall Upper Engine Components
    Reinstall the upper intake plenum covers on both cylinder banks, reversing removal steps. Reconnect all vacuum lines, breather hoses, and electrical connectors in their original positions. Tighten plenum fasteners progressively in a criss-cross pattern (do not over-tighten aluminum components). Reinstall air intake components, airbox, and intake tubes. Secure all clamps and verify proper seating of intake connections.
14. 14

    Refill Cooling System
    Install and tighten the radiator drain plug or close the petcock valve to specification. Begin filling the cooling system through the radiator neck or expansion tank with the Aston Martin-specified coolant mixed to the correct concentration (verify coolant type and mix ratio against the factory manual). Fill slowly to allow air to escape. If equipped with cooling system bleed screws (location must be verified for this platform), open them during filling until coolant flows without air bubbles, then close. A vacuum fill is strongly preferred for this engine. Fill to the cold fill mark on the expansion tank. A vacuum filling tool is highly recommended for V12 engines to avoid air pockets. Note: Begin filling the cooling system through the expansion/header tank with the Aston Martin-specified coolant mixed to the factory-specified concentration (confirm chemistry and system capacity against factory documentation). Note: Begin filling the cooling system through the expansion/header tank with the Aston Martin specified 50/50 coolant (confirm coolant chemistry against factory specification). Note the DB9 does not have a radiator neck fill point.
    Torque spec

    Coolant Drain Plug15 Nm (11 lb-ft)

    Bleed Screw9 Nm (7 lb-ft)
15. 15

    Bleed Cooling System and Check for Leaks
    With the expansion/header tank cap removed, start the engine and allow it to idle. Monitor coolant level and add as needed as air is purged from the system. Run engine until it reaches normal operating temperature and the cooling fans cycle on. Note that fan activation is ECU-controlled by coolant temperature and does not by itself confirm thermostat opening. Rev engine gently to 2000-2500 RPM several times to help purge air. Observe the bypass pipe area and all hose connections for leaks. Once fans cycle and no air bubbles appear in expansion tank, shut off engine, allow to cool slightly, and top off coolant to the hot fill mark. Reinstall radiator or expansion tank cap. Reconnect battery negative terminal and reinstall engine covers and undertray.

Reassembly

1. Reinstall all components in reverse order of removal
2. Ensure all vacuum lines and electrical connectors are reattached to their original locations
3. Verify all hose clamps are properly positioned and tightened to specification
4. Double-check that no tools or shop towels remain in the engine valley before closing up
5. Reinstall engine covers and undertray with all fasteners properly secured

Verification

• Start engine and verify no coolant leaks at bypass pipe or hose connections
• Allow engine to reach full operating temperature and confirm cooling fans cycle normally
• Check coolant level in expansion tank after engine cools—top off if necessary to cold fill mark
• Monitor coolant level over the next few days and top off as air pockets continue to purge
• Verify no error codes or temperature warning lights on instrument cluster
• Inspect engine valley and ground beneath vehicle for any signs of coolant seepage after test drive

Related trouble codes on this vehicle

Codes that commonly send this job to the bay — tap one for symptoms, causes, and diagnostic steps.

Chasing one of these codes and not sure which part is guilty? Ask a master mechanic about YOUR car →

More procedures for this vehicle