Hi gang,

Long time lurker, seldom poster here. I've searched the forum and can't find a match to my case so I thought I would post the question to see if anyone can help.

I have a 2015 A6 Avant, 200kw, 3.0 tdi with about 100k on the clock. Never missed a service and has always worked like a charm. I am not the first owner but have put the last 30k or so on it. It came to me with FSH (not all main dealer). 

Now a few weeks back I was on a relatively long and spirited drive back home from Oxfordshire when I noticed the engine was overheating (warning light on and torque limited). I pulled over and immediately upon slowing down the car started to cool down again. I checked the expansion tank and I wasn't low on coolant. As the car had cooled down again I thought I'd just drive a bit more judiciously for the remainder of my journey which worked just fine. A couple of weeks after that, I was driving to work at motorway speeds and noticed the car heating up, this time only by a couple of LEDs over 90c on the dial. Again, it cooled back down to 90c when I slowed down to about 50mph and cruised like that for a while. This seemed to be getting more frequent so I took it to my mechanic. As the car never overheats on idle and only overheats when driving at speed, he suggested it was the water pump as opposed to the thermostat. He changed the water pump for me but the problem still persists on journeys over about 25 minutes in duration. Even stranger, last night I went for a drive to confirm this, as I hadn't had a longer journey for a week or so, the engine heated up as described above. I then put the cabin heater on full blast and HI heat for a while and the temperature came down rapidly and stayed at 90c regardless of my speed. 

All very peculiar! 

Has anyone experienced this before or does anyone know of what could be causing this issue? I've spoken to an Audi specialist who thinks either the new water pump was faulty out of the box or the thermostat might actually be the problem, even though the symptoms aren't typical (if such a thing exists). I did speak to him before my experiment last night though, so he doesn't have those details. I'm at my wits end trying to figure this out so any help and guidance from those in the know here would be much appreciated!

memzey

Turning cabin heater on full speed on max temp was trick used years ago when the thermostats stuck shut and used the heater matrix to cool the engine instead of your radiator. If doing this prevents your over heating then you may have found your problem. Also worth checking is active grilles, does your car have them and are they working. Should get a fault if they fail but have seen it before. They usually cycle when the car is switched on if fitted but you would need someone to watch for you. 

Any good reason not to renew the thermostat on this 10. year old car, when the symptoms are not untypical of a falling one? 

Thanks guys. Not sure if my car has active grills, is there an easy way to check? I too am leaning towards the thermostat so glad, in a way, that you agree. I’ll chat with my mechanic and take it from there. 
 

One last question on this, if I may, is there a non-oem brand for this part that is recommended or is it something that needs to be main dealer? The disparity in price between oem and aftermarket is large and one I’d like to avoid if possible. 

Any reputable brand should be fine, Audi don’t make these parts themselves anyway so possible you get the exact same item but in a different box. Active grills are visible looking thru the front grilles of the car. 

Ok, I’ve just dropped it off to the mechanic. I’ll update this thread when the car is back and I have had the opportunity to check if the new thermostat has resolved the issue. 

Picked the car up yesterday and, touch wood, so far all is well. I gave it a good 30 minute high rev blast to force DPF regen and the heat didn't deviate from optimum at all. Previously it definitely would have overheated from such driving. The radiator fans are also having a far easier time of it which is always good and quieter. We went with a thermostat from the main dealer in the end. Time will tell if this works long term but so far so good. 

Glad to hear a new thermostat resolved the problem. A shame you were talked into a new water pump before investigating the simplest solution.