Coolant Flush

Complete coolant system drain, flush, and refill procedure for the M275 6.0L V12 BiTurbo engine, including air bleeding to ensure proper cooling system operation.

Warnings

Tools required

Parts

• Drain plug seal washer × 1 — Use OEM specification
• Coolant temperature sensor O-ring (if equipped) × 1 — Use OEM specification

Fluids

• Mercedes-Benz Coolant (Blue) — 18 qt

Preparation

1. Ensure engine is completely cold (below 100°F / 40°C)
2. Park vehicle on level surface and engage parking brake
3. Remove engine cover panels to access coolant expansion tank and bleed points
4. Locate coolant expansion tank in right rear of engine compartment
5. Position drain pan underneath radiator drain point at lower driver side
6. Have all 18 quarts of new coolant premixed to 50/50 concentration with distilled water ready

Procedure

1. 1

   Relieve cooling system pressure
   Slowly turn the coolant expansion tank cap counterclockwise to the first detent and allow any residual pressure to escape. Wait until hissing stops, then remove cap completely. Inspect cap seal for damage and replace if necessary.
2. 2

   Drain coolant from radiator
   Locate drain plug at bottom driver side of radiator. Place drain pan underneath. Remove drain plug and allow coolant to drain completely. This will take 10-15 minutes for the V12 system. Inspect drain plug washer and replace if damaged.
   Torque spec

   Block Drain Plug25 Nm (18 lb-ft)
3. 3

   Drain coolant from engine block
   Locate drain petcock on lower section of engine block on both cylinder banks (left and right sides). Open both petcocks by turning counterclockwise. Allow complete drainage. The V12 has separate cooling circuits requiring drainage from both sides.
   Torque spec

   Drain Petcock5 Nm (4 lb-ft)
4. 4

   Flush cooling system
   Close all drain points finger-tight only. Fill system with distilled water through expansion tank. Start engine and run at idle with heater on maximum until cooling fans cycle on and off twice. Shut off engine, allow to cool 30 minutes, then drain completely again. Repeat if old coolant was contaminated.
5. 5

   Close all drain points
   Install new drain plug washer on radiator drain plug. Reinstall and torque drain plug to specification. Close both engine block drain petcocks and torque to specification. Ensure all drain points are secure before filling.
   Torque spec

   Block Drain Plug25 Nm (18 lb-ft)

   Drain Petcock5 Nm (4 lb-ft)
6. 6

   Locate and prepare bleed screws
   Identify all bleed screw locations on the M275 engine: one on each thermostat housing (left and right cylinder banks), one on heater core supply line, and one near expansion tank. Have 8mm wrench ready for bleed screws.
7. 7

   Begin coolant fill with bleed screws open
   Open all bleed screws by turning counterclockwise 1-2 turns. Slowly fill expansion tank with premixed coolant using funnel. Fill until coolant begins to seep from lowest bleed screw. Close that bleed screw to torque specification, then continue filling until next bleed screw seeps. Repeat for all bleed points working from lowest to highest.
   Torque spec

   Bleed Screw8 Nm (6 lb-ft)
8. 8

   Fill to proper level
   Continue filling expansion tank to the MAX cold fill line marked on tank. Ensure all bleed screws are closed to specification. Install expansion tank cap hand-tight only at this stage.
9. 9

   Initial engine run and manual bleed
   Start engine and allow to idle. Monitor coolant level in expansion tank and add as needed. Run engine until both cooling fans activate (approximately 190°F / 88°C). During warm-up, open each bleed screw individually for 2-3 seconds to release trapped air, then close to specification. Watch for air bubbles in expansion tank.
   ⚠Monitor temperature gauge closely. If temperature exceeds normal range, shut off engine immediately - this indicates air lock.
   Torque spec

   Bleed Screw8 Nm (6 lb-ft)
10. 10

    Perform diagnostic tool air bleed
    Connect Mercedes-Benz Star Diagnostic Tool. Navigate to Engine Control Module > Actuations > Coolant Air Bleed. Run automated bleed function which will cycle coolant pump at high speed for 10 minutes while modulating thermostat positions. Monitor for coolant level drop and refill as needed. The V12 requires this automated procedure for complete air removal.
11. 11

    Final bleed verification
    After automated bleed completes, run engine at 2000-2500 RPM for 5 minutes. Both cooling fans should cycle normally. Check all bleed screws one final time, opening momentarily to verify no air discharge (only coolant). Close all bleed screws to final torque specification.
    Torque spec

    Bleed Screw8 Nm (6 lb-ft)
12. 12

    Set final coolant level and pressure test
    With engine at operating temperature, shut off and allow to cool 15 minutes. Check expansion tank level and adjust to MAX line. Tighten expansion tank cap fully. Restart engine and verify no leaks at drain points, bleed screws, or hose connections. Check for pressure buildup in expansion tank (cap should be firm when pressed).

Reassembly

1. Reinstall all engine covers ensuring proper fitment
2. Wipe down any coolant spills from engine bay to prevent future corrosion
3. Dispose of old coolant according to local environmental regulations

Verification

• Run engine through complete heat cycle with cooling fans activating at least twice
• Verify no leaks from any drain points, bleed screws, or hose connections
• Check coolant level after engine cools - should remain at MAX line with no significant drop
• Confirm heater output is hot on both driver and passenger sides indicating proper circulation
• Use diagnostic tool to verify no coolant temperature fault codes stored
• Test coolant concentration with refractometer - should read 50/50 mixture (-34°F / -37°C freeze protection)
• Monitor coolant level over next 50 miles and top off if needed - minor settling is normal

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