Coolant Reservoir overflow
 

My coolant reservoir is overflowing about when my car's temp hit's 225 F. It just pees out the back. I checked it for cracks, and I couldn't seem to find any. I thought maybe my thermostat was bad, but coolant appears to be circulating since my radiator gets hot. Without fail, whenever my engine hits 200 F, my coolant pressure notice goes off. This initially led me to believe I had a problem with my thermostat. That doesn't make sense to me though if the radiator is hot. How is the radiator getting hot if the thermostat doesn't open? Also, around 225, my coolant starts to boil, which should be impossible since the pressure of the system ought to raise the boiling temperature to 362 F.

Anyways, my thoughts so far are that it can't be the thermostat since the radiator is getting hot. I'm leaning in the direction of the coolant tank.

Also, it's my understanding that the cap is supposed to lift and that's how coolant is supposed to leave the tank. Not by exploding out the back... Am I right? The service manual didn't indicate any "safety valves" on the coolant reservoir.

In addition...
 

Is it possible to replace the thermostat without removing the timing belt as per the Bentley? If it ends up being the thermostat, I'd rather like to avoid removing the timing belt.

Waterpump, thermostat, or air bubble trapped. Coolant temp shouldn't go above 190F normal with 200F the high point.

you can certainly replce the thermostat without the timing belt but you are about 80% of the way there in labour in my opinion. cant recall if you can do the water pump but i did it all in my 2.8 twice. i guess theorhetically you could compress the tensioner to release the timing belt ( it drives the water pump ) do the water pump and then put it back ... sounds nightmareish to me tho

Initially this is what I thought. But, the radiator is getting hot and fluid is clearly circulating. After my first blowout of coolant, I filled the tank with water. The water, which was obviously clear, turned pink.

I consulted with a friend of mine that used to be an Audi mechanic. He believes that my reserve tank is cracked somewhere, or that the cap isn't holding pressure like it's supposed to. The part arrives today, so time will tell.