So we had a good chilly morning here in CT this morning (~10f), and about 5 minutes into my regular morning commute I realized that despite the engine being at normal operating temp, I had NO heat. Zero. Not even a hint of warmth. I tried fiddling with the controls to see if I could get something to reset, but had no success. After a few more miles, I could no longer feel my fingers or toes and thus decided to bail on class and head back home to troubleshoot. About this time, I caught a red light out of the corner of my eye - Wouldn't you know it was the temp gauge, just reaching maximum. I pulled over and shut down, popped the hood, and dumped some snow on the engine and rad the help cool things down. Oddly, the rad wasn't even warm enough to begin the melt the snow. After about 5 minutes, the temp gauge was back to normal and I continued on. All was well for a minute or two, and then water temps took a hike back into the red. At this point I was in heavy traffic and only about 2 minutes from home, so I continued cautiously with the gauge just under red until I reached the driveway (I'm hoping this wasn't enough to warp the head, especially considering ambient temps right around 10f).

History: I know I have a coolant leak, presumably at the overflow bottle. I just topped it off last week, but I would not be entirely surprised if it was low already. However, I usually get a code and / or increased hi-speed fan activity when it gets low, and I had none of that yesterday. And of course, I've never seen elevated temperatures as a result of low coolant either, even on much hotter days. Additionally, I know the low-speed fan is a known issue that can cause overheating, but the lack of core temp at the rad and the simultaneous failure of the heating system, in addition to the fact that this all happened while at speed on a FREEZING cold day, suggest a failed thermostat to me. Does that seem correct?

 

Yep, bad t-stat.

 

Agreed.

 

The fact that your cooling system has been low a few times, and topped off....I would also check the freeze protection of your coolant, ensure that the air has been purged from it, and that you have added only compatible coolant to what you are currently useing.
All of this is really a bit of a moot point, since it appears that your thermostat has failed, needs to be replaced, and the above actions will be done as part of the repair!!

 

Hmmmm....

I'm almost certain it's the thermostat that's stuck closed, not allowing coolant to circulate. However, most cooling systems are designed that the heating system always works as long as the water-pump is still spinning. At least, this was true with my '81 Audi 4000, and more recently with my '01 VW New Beetle which had a stuck (closed) thermostat last summer. I turned the heat to maximum & fan to highest speed and was able to make it home in 90*F ambient temperature. Basically I used the heating core & fan in place of the radiator (which was completely cool).

The Mini's cooling system could be routed differently in that if the thermostat is stuck closed, no coolant will be circulated to the radiator as well as the heating core... which, if true, would be a stupid design of the first order. Without the Bentley manual at hand, I cannot confirm this.

Other possibilities are:

1. Not enough coolant in the cooling system (as you indicated there's a leak).
2. The water-pump is dead... which you would know since it's driven by the S/C. The PTO gears would make a lot of noise before it's completely dead.

Let us know what you find out...

 

...or the cooling system was blocked by a snowball, due to insufficient freeze protection for the tempurature...

if that is the case, the symptoms might just go away, but I would be very attuned to potential side effects of local hot spots in the engine due to abnormal cooling system performance -- e.g. head gasket problems.

you definitely do not want to see any slick on top of the cooling overflow tank, nor any grey scum on the dipstick.

i'd bet you're OK, because you were paying attention and caught the engine overheating immediately when it occurred.

fingers crossed,

Charlie

 

I had thought of a localized freeze-up due to the fact that (1) I have been topping off with straight water for ~6 months and have likely diluted the AF a bit, and (2) it was really, REALLY cold out. But ultimately, I couldn't bring myself to believe that there wasn't enough core temp in the engine compartment to melt whatever blockage may have occurred. Furthermore, wouldn't the additional pressure of a blocked system have manifested itself somewhere?

Still the lack of interior heat might suggest blockage - As Cadenza said, it doesn't make much sense for the heater core to be on a thermostat-dependent circuit. Can anyone familiar with the routing of engine coolant confirm or deny whether this is true?

 

If I were you, I'd toss a new thermostat on, since they leak, and it is likely yours is and it is likely causing your coolant loss. Thn refil with fresh BMW/mini coolant...the blue stuff....and you are only usind distilled water right? I would hate to see you block up a radiator with lime scale. Then if the loss continues, swap out the coolant tank. Those are the two most likely causes. Talking about why what happened will not fix it, and if it happens again, you will possibly need a head-gasket, if you don't already need one from your overheating experance. Act now. Sure, maybe you did freeze water/weak coolant in the motor...maybe you did not. But you know you need fresh coolant to get rid of the weak crap in the car, and you have a tiny leak some where.....fix it now. If you put it off, the $ may turn into $$ and then into $$$$, and a new motor. You would not be thr first!!
Next time it freezes, it could crack the block.....the head....or who knows. None good, and it is friggin cold out!!