The car is a 2004 A4 1.8t. It has an obvious coolant leak somewhere. The coolant level in the reservoir goes below the minimum mark after every 300-400 miles.

One day I was driving the car and noticed the temp gauge started moving past the mid point slowly. The low coolant lamp came on as soon as the gauge was nearing the red zone. So I quickly pulled the car over and turned it off. Thinking that it was just low on coolant, I opened up the hood preparing to fill the reservoir up (yes, I know it's a really bad idea to do that while the car is hot), but the level was already near the maximum mark. That same day, I noticed the heater inside the car was not working. Only cold air. I believe the two issues are some how related.

I barely made it back home that day. I drove the car in 2 minutes intervals (with a lot of coasting), in between multiple 10 minute cool off periods (with the engine off).
Does anyone have any ideas as to what can be causing this over heating?

 

if its not leaking (visibly) check out the coolant flange. its usually a culprit of coolant consumption in these cars

 

I'm pretty sure i've seen traces of coolant on the ground before. The problem is, the coolant reservoir was full and it was still overheating.

 

remember that your heater core acts as a radiator as well. if you ever experience overheating, turn the heater on. If you are not getting any heat, then the heater core may be clogged. First thing that you need to do is find the coolant leak, then you need to flush the entire cooling system and see if that helps. You may be having issues with the coolant/water mixture not being at a correct ratio. 50% coolant / 50% distilled water.

Heater core flush DIY:
http://www.audiworld.com/tech/eng115.shtml

Just because the coolant reservoir is full does not mean that your cooling system is full. There may be a clog somwere.. flush the entire cooling system and report back.

 

I had a similar problem. The car would overheat in start and stop traffic, fine on the freeway. Turned out that the ECU was sending erratic signals to the fan control unit (FCU), which was turning the radiator fans off as soon as it turned on. Burned out 3 FCUs before I figured it out. Replaced ECU, FCU, and the fans/motors which had burned out. Total PITA.

 

DSM, did you ever find your answer??? I just replaced the coolant flange which was the biggest PITA project EVER (I would rather do another engine swap, FYI).... But I may be having some of the same issues...