Some history first. I have a 00 A4 B5 1.8TQM. It has a brand new thermostat, as I had less than stellar heat this fall, and my temperature gauge would not climb above minimum unless my car idled for half an hour or more. The new thermostat and a coolant flush fixed this problem, and I've had very good heat since. Until the last two days...

It's been -35C here the last few days (-31F)... so cold. Really cold. I've let the car run for 20 minutes or so each morning to let it get nice and warm, and not do any damage. However, upon getting in after the warmup period, the heater is still blowing ice cold air, even though the engine temp is right at normal. It continued like this all the way to work and while driving the engine temp gauge began to climb. In fact, by the time I got to work this morning (10 minute drive) the engine was overheating and beginning to leak coolant. Overheating in -35. Joy!

The first day I was able to shut off the climate control, and turn it back on with only defrost cranked, and suddenly it blew hot air, and my temp gauge went back to normal. After work, the heater worked perfectly. However, this morning, no matter what combination I tried, I couldn't get it to stop blowing cold air, and the engine temp skyrocketed quickly.

Obviously the car thinks it's unloading heat from the coolant, but it isn't getting to me in the interior, and the hot coolant is continuing back in to the engine, causing it to overheat. What I can't figure out is where the issue is. If it's the heater core, why did it suddenly start blowing hot yesterday and return to normal? I can't see it being a bad t-stat, as the temp gauge is working fine.

I'm confused.

 

Not about your heating issue but you aren't doing your car any favors by letting it idle for that long. It will warm up much faster if you start driving it after a few minutes rather then letting it idle for 20-30 minutes. It's better for the car too.

 

Cool, thanks for the tip. I was trying not to freeze when getting in to the car to leave haha. You're from Edmonton, you've been out the last couple days!

 

Yea, it's been super cold here compared to the balmy weather we've had the past couple of weeks, and are expecting nice weather again next week! Heated seats are a blessing, between that and gloves I can manage the drive for the few minutes it takes the car to start warming up as long as the windshield is clear.