This Audi is driving me crazy. I just purchased it a month ago. Replaced timing belt,water pump, both coolant sensors, thermostat and expansion tank and cap. Bleed cooling system, now coolant is boiling over in tank. Temp gauge never goes past half way. Just got VCDS, no errors for cooling sensors. I'm totally stumped.Any idea what to check or look into. Pretty positive thermostat is not opening, top hose is hot, lower is cold. With the electronic thermostat is there any way to test? Any help is appreciated. Car is an 05 1.8t with a 6 speed

 

You said that you replaced the coolant pump, but symptoms say non-functioning pump. Boiling coolant in the reservoir says the coolant is not circulating through the head, same for lack of heat detected by the CTS, same for cold lower hose.

 

How often are you bleeding the system? Does it have air in the system after every drive? I dealt with some of the problems you are describing (not all mind you) and I ended up having a warped head between cylinder two and three. I had to take the head into a machine shop and $60 dollars later had a perfectly flat head that is now powering my audi perfectly. Before that my car would boil over due to air in the system after a few miles. If you have a compression tester, see if any of the cylinders are dramatically lower than the other ones. The 1 and 4 cylinders were 30-35 lbs higher than 2 and 3 which ended up being the problem.

 

Also check for cracks between valves if you end up removing the head (might as well change the head gasket while you're there too) If you end up pulling the head, you should replace the rear coolant flange as those warp and leak and cause problems and are relatively difficult to swap with everything in the way. I heard people talking about swapping that flange being the hardest thing to do on the car, so when it didn't take that long for me I was pleasantly surprised and if you get the chance to change it while the head is off, even better and easier.

 

planning on doing a compression test this week when I ge some time. Really hoping it's not a head gasket issue.

 

If you have the original AMB head, it's far more likely a crack has formed or the head has started to warp than a head gasket issue. Blown head gaskets happen, but seem to be less frequent than head issues.