jayada

2011 Q7 3.0 TFSI. Couple days ago on an 85 degree day the AC suddenly started blowing lukewarm air. I looked at the temp gauge and it was at 3/4! It kept rising almost to H and the high temp warning came on. Then the temp gauge dropped all the way to C within one second and the AC started blowing cold air again. three or four minutes later the temp gauge then shot to exactly half way within a second. It then started creeping up again. The cold air turned lukewarm again and the temp gauge crept slowly but steadily again toward HOT!! Later in the day after the car cooled down after a couple hours I drove it home which took about 15 minutes on the highway. The exact same behavior occurred. Temp gauge went and stayed at half then slowly crept to near hot then instantly dropped to C and after a couple minutes instantly went to half way, then crept to hot again with the high temp warning going off. That's about when I got home. Popped the hood and the engine and cooling system was super hot. The fan was blowing with the engine off. Looking under the car coolant was dripping from the pressure release valve. Trouble in paradise.

Yesterday I drove it gingerly and all systems seemed AOK. Took it on the highway and all seemed fine. Temp gauge never went above half way. After 20 minute drive in 85 degree weather everything seems just fine. Engine was warm but felt normal. Engine fan was off...nowhere near as hot as it was during the problem.

I'm thinking it might be a bad thermostat but I don't know. Anyone have any ideas?

Q7-retired

anytime i've had erratic temp swings, it was a leaking head gasket. on other cars, not on the Q7. What color is your oil? if it's cafe au lait, hoo boy.

smith2010Q7

Yes it does sound like a bad thermostat.

7-up

.. i must be cross eyed - not sure what car is being discussed here but I have a 1995 V6 cabriolet- ... with the same. The engine runs dead cold unless the car is static for a bit. I'm certain (as I can be) that in the past some 'unusual mechanic' has done a quick cam belt change by removing the water pump and during which the thermostat dropped out and it was not put back ini!! From the Technical detials i've studied the 'stat' is behind the water pump.
This is also a trick in the trade ...If you have a car with a leaky head gasket , put some super gunge in the oil and water systems - take out the stat and then it never gets hot enough for the gasket to leak (until you have sold the car !!).
Why am I so sure ,,, well thanks to an electronic guru brother we have already rebuilt the dash and replaced all sensors - so yes my car has to be running 'cold' .. and that can only mean the is stat stuck open or not there at all... Hope this helps ...

This Autum I commence cam belt change - and stat replacement

Prime Time

jayada - did you ever get this fixed? My 2012 Q7 is doing the exact same thing.