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Coolant flush on N55 - DIY guide

MMapTuner Pro

@maptuner_pro

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Did a coolant flush on my N55 335i today. BMW recommends every 4 years but mine was 6 years old (oops).

Process:

  1. Drain old coolant from radiator drain plug
  2. Remove thermostat housing to drain block
  3. Flush with distilled water (3 times)
  4. Refill with BMW coolant (50/50 mix)
  5. Bleed air using the bleed screw on thermostat housing
  6. Run engine to operating temp, check level

Old coolant was brown and disgusting. New coolant is blue and clean. Should have done this sooner.

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2 replies

  1. #1
    WWrenchMonkey

    @wrench_monkey

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    Good guide. One important thing to add - the N55 has a bleed screw on top of the thermostat housing. After filling, open this screw and let coolant flow out until theres no air bubbles. Then close it and run the engine with the heater on full blast for 10 minutes. Check the level again after it cools down. Air pockets in the N55 cooling system can cause overheating.

  2. #2
    EEuro Mechanic

    @euro_mechanic

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    Always use BMW-approved coolant (blue) or Zerex G-48. Never mix green and blue coolant - they use different corrosion inhibitors and mixing them creates a gel that clogs the system. I see this mistake at the shop at least once a month.

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