Audiboy

I think you mentioned it was colder at the time, but I think you might have a hotter running engine for whatever reason - incorrect T/B timing or water pump not fully functional. Do you have a vag to check to see if you are running lean or a boost gauge to indicate what your vacuum might be when idling and decelerating?

Here's how I will approach it.

If I have a cooling fan problem,

- then cruising at freeway speeds should cool down the radiator. No fans required.

- Turning on the A/C turns on both fans (maybe half speed), but this should move enough air through the radiator to cool it if you are idling (no load on engine).

Then, if forced air doesn't work, look for blockage on the radiator fins. If none, then it might be water flow through the radiator or no pressure from the pump itself.

On my other cars (except the A4), when I suspected something, I take off the thermostat 1st - and run it straight. If there is no problem, the temp will be really cool since the water will circulate and then be cooled by the radiator before it can buildup any heat. If you have a cooling problem, you will still see the temp rise up. Also, without a thermostat, you can observe lots of water movement on the reservoir - or take off the upper radiator hose and see it all flow out (when it is cold of course).

I haven't fiddled with the TT yet to see if you can run without a thermostat, but if electrical debuging leads you nowhere, maybe it's worth looking into.

sevensixtwo

...then it turned cold outside and the problem didn't reappear until a month ago (warmer temps outside). Right after it happened the first time, I brought it in to get my timing belt/water pump/thermostat replaced - hoping the new water pump/new thermostat would fix the issue.

At idle, the boost gauge shows ~18-20 inHg, but haven't paid much attention to the gauge when decelerating. I'll check that next time I run the car. I do have a VAG-tool - I'll have to look into checking if the car is running lean. I did a quick visual check of the radiator and found no blockage, but I can barely see anything without the front bumper removed - I'll have to check it out more when I attempt to install the new Fan Temp Switch.

Thanks again for trying to help diagnose my issue.

FYI: Weird thing is...I put in the new OEM thermostat last Friday. Tested the car Saturday and Sunday (driving around at least 45 minutes and idling 15 minutes) and my symptoms occurred (not as bad as when the Whaler T-Stat was in though). Since then (same tests Monday and Tuesday night), the car has been gradually running normal temps. Today it's been dead center and temp was sitting at 95 degrees for about 45 minutes of driving an idling. Not sure what's going on - I'm hoping it stays that way. Not sure if they're accurate tests, but I'll have to continue on through the week. Why would a mechanical part like the T-Stat need time to adapt? Or is the ECU or Fan Control Module adapting to the new T-Stat???

sevensixtwo

I returned the car (71500 miles, 2001 TT 225 - out of warranty) to the independent shop that did the timing belt service, hoping they could figure out why the car was overheating (water temp gauge pegged at "H", low coolant/hot engine warning symbol flashing and chiming, fans intermittently turning on and off, and A/C readout telling me car was at 121 degrees Celcius). Unfortunately, they couldn't figure it out.

Reluctantly, I took it to the Audi dealership (knowing it would probably cost a fortune to fix). Taking it to the dealership was my last resort.

Result? Problem fixed within 2 days. Instrument Cluster replaced for just under $1000. I'm happy again (knock on wood). I ruled the cluster out originally because the A/C readout for the coolant temp was at 121 degrees when the gauge was at "H".

FYI, here's a list of items that were changed before I took it to the dealership:

Timing Belt
Water Pump (metal impeller)
2 Thermostats (Whaler brand, then OEM)
3 Coolant Temp Sensors (green version)
Fan Temp Switch
Coolant Flush (twice, with Audi/VW G12 coolant)
New fuses for all associated cooling parts

Thanks to those here for trying to assist me!

sevensixtwo

...my car wasn't really overheating. The cluster was failing making the car look like it was running too hot. The cluster even messed up my A/C readout for coolant temperature.