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Thread: Engine suddenly running cooler?

  1. #1
    Join Date
    Oct 2005
    Posts
    518

    Default Engine suddenly running cooler?

    Hello everyone,

    As the title states, my engine has suddenly decided to run a bit cooler than normal. Ambient temperature is about 100 degrees, and even in traffic, the gauge does not reach the 1/2 way mark as it used to before. I've also noticed that the fan doesn't engage as much as it used to, even in traffic. During warm up the gauge indicates a pretty normal temperature increase from cold to about the 1/4 way mark, after which it ends up about half way between 1/4 and 1/2 way points on the gauge and seems to prefer to stay there. At speed, the temperature tends to decrease slightly, as it has done in the past (constant air flow through radiator). The coolant level is normal, the expansion tank cap is new (so is the tank), and in general the system seems healthy. Although I like to see the engine running cooler in this ish weather we are having, I'm a little worried about its sudden change in behavior. So far I have only two plausible theories:
    1) The thermostat is stuck open, and that's why the engine doesn't increase temperature as rapidly. This doesn't explain why its running cooler in general though. (The thermostat is 2 years old, temperature spec unknown)
    2) There was a restriction or a clog somewhere and suddenly it is gone. More flow=better cooling. (The coolant is 2 years old, BMW blue)

    Is there anything else that can cause this type of sudden change?
    Thanks in advance for any input.

  2. #2
    Join Date
    Mar 2004
    Location
    NSW, Australia
    Posts
    445

    Default

    The spring on your thermostat probably just broke. Common occurance.Pull it out and check it.

  3. #3
    Join Date
    Oct 2005
    Posts
    518

    Default

    While on the subject of thermostats, what is the stock temperature rating for the M30?

  4. #4
    Join Date
    Aug 2006
    Location
    Boulder, CO, USA
    Posts
    195

    Default

    RealOEM shows an 80C thermostat. My guess would put the thermostat temp either at 80, 88 or 92C.

  5. #5
    Join Date
    Oct 2005
    Posts
    518

    Default

    Quote Originally Posted by colo525i
    RealOEM shows an 80C thermostat. My guess would put the thermostat temp either at 80, 88 or 92C.
    All of the thermostats I see available from aftermarket sources are either 80, 75, or 71 degree. Would a broken thermostat cause the cooler temperature in traffic situations? That is what's puzzling me, since I'm aware of what should be happening during warm up with an open t-stat.

  6. #6
    Join Date
    Apr 2006
    Location
    Chicago, Il. U.S.A.
    Posts
    4,243

    Default

    Your thermostat is broken. Be happy it failed in the open position.
    80c stat, an o-ring and you're good to go.
    "The gas pedal wouldn't go to the floor if it weren't meant to be there"

  7. #7
    Join Date
    Jul 2006
    Location
    El Paso TX
    Posts
    1,534

    Default

    It certainly could since the fan is mechanical, not electric. The T stat is the only thing that holds the car to operating temp instead of letting it run cooler.

  8. #8
    Join Date
    Jan 2004
    Location
    Albuquerque, NM
    Posts
    953

    Default

    Ditto, best to change the thermostat asap. My temp gauge started drifting randomly for a period of about 2 weeks before the thermostat failed in the closed position. Normally they fail in the open position. Of course Murphy's Law guaranteed mine would fail closed while on a road trip, 100's of miles from any source for a new one. Try finding BMW parts in Bismarck. If it ain't Ford, Chevy, or Dodge, they don't have a clue what you're driving.

    m30 temps:

    gale
    92 735i 5-spd, turbo project finally underway!


  9. #9
    Join Date
    Nov 2004
    Posts
    205

    Default

    was wondering... can the thermostat be adjusted? mine is running a needle above the middle point?

  10. #10
    Join Date
    Oct 2005
    Posts
    518

    Default

    Thanks for everyone's input. I went ahead and ordered a new 80 degree thermostat and new radiator hoses. Its about time for a cooling system service anyway. Currently my engine likes the region between 144 and 153 degrees on Gale's diagram.

    And pong,
    You can't adjust the thermostat. If you are running past the middle, you may need to take a look at your fan clutch, radiator condition, and coolant level. Getting a lower temperature t-stat also won't change your final running temperature, but it will affect how quickly the engine warms up.

    Once again, big thanks to the collective mind of bimmer.info

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