Okay so I recently purchased a 2001 A4 1.8TQM engine code AWM. Timing belt and water pump done about 20k miles ago.
When I first got it, the car would not get hot at all so I attributed that to a thermostat stuck open. I changed the thermostat, along with the housing, coolant temp sensor, coolant flange, expansion tank and cap. After, the car began to heat up.l and I saw no leaks anywhere. Everything seemed normal until I began to drive. The car would get slightly hot, maybe 1 line past halfway. I then tried to bleed the system thinking there was air in it. The same thing happened again. The engine seemed really hot to the touch as did the upper radiator hose. However the lower radiator hose was ice cold. I even unbolted the housing and let the antifreeze come out and it was ice cold. I then took out the thermostat and the antifreeze on that side was very very hot. So, I'm thinking, the thermostat isn't opening right? So I deemed that the thermostat was defective (since I bought it from Autozone), and went to the dealership and got an OEM thermostat. Now the temp gauge doesn't go past halfway, but the lower radiator hose still seems cold. I opened the expnansion tank cap just a short time ago and it started bubbling out. (FYI, I only have straight water in the system for now as I did not want to keep wasting money on antifreeze until I figured this out. I'm thinking that the bubbling I just talked about is the system boiling over because of no coolant movement so everything sitting in the engine block just boiled over. But the temp gauge never went past halfway!?!? WTF!)

As for now, I'm left thinking
1. Air in the system
2. Bad water pump that isn't flowing (but the car never overheated with the original thermostat that was stuck open....)
3. Is it possible that I got two defective thermostats with the second even being OEM? Yes I tested them and they opened in boiling water.
4. Do I want to set this car on fire and watch it burn? Lol

I'm at a loss for words as to whats going on. I've done a thermostat on my last B5 and had no problems. Also, the Aux fan (passenger side) does not ever come on, not even when I turn on the a/c. I'm assuming that since the lower radiator hose is cold (where the aux fan switch is located), that is obviously why.

Also, after assuming the first thermostat was defective, I put the original thermostat that was stuck open back in and everything went back to how it was originally (never heats up).

Or am I crazy and nothing is wrong at all and the lower radiator hose just stays cold?

 

The temp guage should never go past half way. Not sure if we are on the same page, but the gauge should always be in the middle once it is up to operating temp.

How about what kind of heat do you get in the cabin?

The lower radiator hose should be warm. Audi's are a little different. Coolant travels up through the radiator instead of downwards.

Without knowing more, I would suspect air in the system. Pull the passenger side heater core hose off a little to expose the bleeder hole.

I will think a little more about this tonight.