The car is 2001 BMW 330i. I have changed ignition coil, spark and IAC. Last week, my car experienced rough idle. Then I changed the coolant temp sensor today. After adding coolant and bleeding, the car ran smoothly in the day time (about 2 hours) without rough idle. I drove the car out tonight. The low coolant light turned on. After driving the car for five minutes with coolant light on, rough idle came back. After restarting the engine, the car became normal again. Any clue for fixing this notorious rough idle issue for E46?

Additional issue: The heater blows only cold air when the car is idle. (I live in Los Angeles. Never use heater.) This might be a sign of bad thermostat. Will this cause rough idle? In theory, bad thermostat might prevent coolant circulating and cause overheating of the engine.

  • 1
    Did you continue driving with the low coolant light on? Was the coolant low?
    – HandyHowie
    Commented Nov 15, 2015 at 11:44
  • Yes, I did. I checked in the next morning. The coolant was low. I bleed the system again, and found more air bubble came out.
    – semibruin
    Commented Nov 15, 2015 at 18:00
  • If the engine was up to temperature, A faulty thermostat would not stop the heater getting hot. Some cars have an electric pump for circulating the coolant around the heater matrix, maybe this pump is it working on your car, if it has one.
    – HandyHowie
    Commented Nov 15, 2015 at 18:47
  • Does the engine temperature stay in the 'normal' range all the time?
    – HandyHowie
    Commented Nov 15, 2015 at 18:48
  • It always reads "normal", even when the coolant light was on.
    – semibruin
    Commented Nov 15, 2015 at 18:52

3 Answers 3


You have air in the system. When you get a lot it's hard to bleed all of it out. Need to run engine with bleed screw all the way off so the coolant can really flow out. Pour 50.50 mix in fast. It's gonna get everywhere. You need to make a large enough pressure difference to push the air out.

Theory says the air SHOULD go to the top, but you probably have a hairline crack in your expansion tank causing air to be drawn into the cooling system everytime you shut the car off after its warmed up. This is cus positive water and air pressure when running is much over atmospheric pressure then when you shut ithe off the pressure rapidly decreases in the system, until it's balanced with atmosphere by air from atmosphere going into cooling system through crack. I had this problem. I'd have to floor it for a few seconds to get enough coolant to circulate through heater and get heat in interior.


Sounds like Cody's answer is fairly correct. You probably just have air in your cooling system. Sometimes they can be a pain to bleed after you have done a repair that has allowed air to enter the system. However, I think there is a better method to remove the air from the system.

Your vehicle is equipped with a heater control valve that is also a small auxiliary coolant pump. It is important to have your heater on high anytime you attempt to bleed air from the cooling system on your car.

  1. Open bleeder valve (located near where you add coolant)
  2. Slowly fill expansion tank with coolant till coolant starts escaping out the bleeder valve.
  3. Shut the bleeder valve.
  4. Turn key on (engine off) with heater set to high.
  5. Slowly add coolant to expansion tank till system wont take any more coolant.
  6. Start engine and take a short test drive till the heater starts to blow warm.
  7. Allow vehicle to cool. Then top off coolant.
  8. Drive vehicle again till heater blows hot and vehicle reaches full operating temp.
  9. Allow vehicle to completely cool down.
  10. Top of coolant one final time.

After following this procedure you should have removed all the air from the system without making any large messes. It is important to keep the following in mind while performing this bleed procedure. ALWAYS keep a close eye on the temperature gauge! If your vehicle starts to run hot. Turn the vehicle off. Allow it to cool. Top of coolant. Then start right back where you left off.

Finally...NEVER open the cooling system when it is warm. You have a nice face... lets keep it that way.


Just to as a couple E46 specifics to the correct answers from Cody and L. hawes:

  • If you can lift the front of the car a little that can be a big help on this car.
  • Hit the front window defrost button on the HVAC unit when bleeding, that will help open all the paths to get the air out.
  • Remember that the temp gauge on an E46 is mostly fiction. There is a huge range where it will point right in the middle. Once it goes toward hot you are really getting hot.

I have bled them for years that way it it works fine, even if it is fiddly. One thing if you are interested in acquiring tools some fancier OBD diagnostic tools will actually let you run a purge via the on board computer. I have the OEM version of this tool: https://www.ecstuning.com/b-schwaben-by-foxwell-parts/professional-bmw-mini-scan-tool/014020sch01a~scf/ and it does an OK job.

It is not necessary, but it is kind of cool to do it the "correct" way.

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