I have an issue I'm struggling to pin down with my 2010 318d 3 Series Touring: All air vents blow cold or at best, luke warm air into the passenger compartment, even on long journeys when the vehicle is at operating temperature. I've had the car diagnosed and done some digging myself and I'm completely stuck as to what the problem could be.

I've attempted to aggregate all the information I can about the car, potential issues and checks made for each one. I must be missing something. Any advice on how to proceed would be fantastic.

Vehicle Details:

BMW 3 Series Touring E91 318d Production Date 08/2010.
150,000 miles on clock.
Engine code N47T.
VIN: WBAUT32000A938336.


Air vents blow cold, barely warm air, even when max heat is requested and vehicle is driven hard.**


  • When set to 28 degrees, HVAC system blows luke warm air into car.
  • OBD (ETC) coolant temp reads 85 degrees C during normal driving/idle.
  • Car does not overheat/run cold (although milage is slightly lower than expected, MPG (UK) in high 40s, my old E46 320d ran in mid 50s)
  • Coolant lines to and from heater core matrix are hot (almost too hot to touch) at operating temp.

Possible Causes & Investigation

1. Broken water pump/broken impeller?

  • Removed aux belt, checked for play in water pump pulley, no play.
  • Car running, removed expansion tank cap, coolant flowing into tank via small hose between expansion tank and top hose. ("fork to expansion tank 17127803838")
  • Car doesn't overheat, no noise from water pump. Coolant appears to be following correctly.
  • -> Water pump appears to be working correctly. Necessary to remove to inspect impeller?

2. Faulty Thermostat?

  • Car runs up to 85 degrees C ETC according to OBD scanner. Holds operating temp at idle and during normal driving.
  • Bottom radiator hose remains cold until engine is warm. Once warm, bottom hose heats up.
  • -> Stat seems to be working correctly. Replace as preventative maintenance anyway?

3. Heater Matrix Blocked?

  • Inlet and outlet pipes to heater core matrix get hot.
  • Followed the guidance in the following pdf regarding flushing the heater core matrix with a hose: https://www.bimmerfest.com/forums/attachment.php?attachmentid=582821&d=1480064170
  • Water flows both directions through the heater core easily. Small amount of sand came out during flushing
  • -> Could be partially blocked still. Could have radweld or other blocking flow through majority of channels?

4. Coolant Low?

  • Front of car jacked 60cm, engine run up to 85 degrees, coolant topped off and bled. Coolant level checked when car level and cold.
  • -> Coolant level is fine.

5. Coolant system Airlock?

  • See above, system bled thoroughly.
  • -> Airlock unlikely.

6. Faulty HVAC Mixer Flaps/stepper motor/s?

7. Faulty Auxiliary Water Pump?

8. Faulty Changeover valve /heater core matrix solenoid?

9. Faulty Electric Auxilary Heater

  • https://www.realoem.com/bmw/enUS/showparts?id=UT32-EUR-08-2010-E91N-BMW-318d&diagId=64_1454
  • Part name, quantity, (date), part number: Auxiliary heater 1 03/2010 64119194595
  • According to JMF Motor Worx, the car is flagging a code for a faulty electric auxiliary heater.
  • Vehicle appears to be specced with the heater (although some forum articles claim that UK vehicles aren't specced with them).
  • -> Shouldn't affect heat when engine at operating temp, even if faulty. Fault might be causing other HVAC issues.

10. Faulty temperature sensor in HVAC system internals.

11. Faulty Centre Console Climate Control Module

12. HVAC - Faulty temperature adjustment dial between middle dash vents. (Under DTC/lock buttons)

13. Faulty Recirculation sensor, under bonnet.

Other Information

Any suggestions of how to proceed would be greatly appreciated.

  • 1
    Welcome to Motor Vehicle Maintenance & Repair! Are the temperatures noted above in C or F? Dec 15 '20 at 17:50
  • Degress Celcuis
    – Phil
    Dec 15 '20 at 17:54
  • Diesel engines are stupid when it comes to the thermostat. Just because the engine isn't over heating, it doesn't mean the thermostat is working correctly, especially in the winter time. I'm not sure about your Bimmer, but many diesel (non-commercial) engines bypass coolant past the thermostat to make it to the heater core. If the thermostat isn't flowing once heated up, or not flowing enough, the core will never warm up enough to give off heat. This is where I'd personally start looking. You can most likely pull the thermostat and test it for proper operation. Dec 15 '20 at 18:05
  • Interesting. My understanding was that the thermostats usually fail open, i.e. the engine would run cold, however the stat remains shut until around 85 degrees C (specified opening temp) and controls coolant temperature nicely. Could the thermostat still be at fault here?
    – Phil
    Dec 15 '20 at 18:36
  • What I described above is how I've experienced it with diesel engines. I mention the non-commercial as not included, because I don't quite remember on them (I'm talking ones like Cummings or CAT diesel engines for 18-wheelers). I have directly experienced this with smaller diesel engines, though. Dec 15 '20 at 18:50

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