Unfortunately I can't help yet as I still haven't sussed this :-(

As far as I can tell only reason for running cold is faulty thermostat. If the thermostat is working correctly coolant does not go through radiator and fans can work at full blast it will still warm up properly.

Another possibility is that it is just reporting temperature incorrectly. Not sure about 3.0 TDI but many cars have several(2) temp sensors.

You are right, the radiator is getting warm, and it shouldn't when the engine is not reaching the 84˚c marked on the thermostat.

Today, I cleaned the engine and went for a blast in the countryside in sports mode to dry it out. The gauge never went past 60˚C and was happier at just 50˚C.

When I got home and opened the bonnet it was barely lukewarm, not the usually heat radiation you feel on your face from healthy cars and some parts such as injector pipes were stone cold. I used an infrared thermometer to check various parts of the engine, the hottest were about 57˚C apart from the EGR pipe and turbocharger shield which aren't cooled. If it was the water pump, it would definitely overheat and I think would show a high temp on the gauge as the coolant temp sender is in a place where this would be detected.

The engine looked very clean, finished a few bits off with brake cleaner and used some Tectane engine lacquer. Looks like new now.+++

I think I'll buy another thermostat, it's a simple 15 minute job the second time round.

I'll return the old one if it makes a difference. I know I could test this one in a bucket of scalding water, but with the slight hassle of removing it, I'd rather just put a new one on while it's off as it was quite cheap from TPS.

My other car (Focus) was running very cold the other day and giving pathetic output from the heater, so I had the thermostat replaced which fixed it.
(didn't do it myself and had a few other minor things done at the same time by my tame mechanic)

But looking at the old thermostat you could see that the central moving part was not moving true and was catching on one side on the body.
I did some tests with boiling water and discovered that it was basically catching so staying stuck in one position, though it could be freed by wiggling or a rapid temperature change.
I suspect it had been misbehaving for some time but it needed a nice cold snap to make it obvious.

Well. I don't know what it can be. I bought another thermostat today and thoroughly inspected and tested it in a saucepan. It opened when the water was about 90*c. I just fitted it after work and refilled and bled the coolant with the 3 bleeding points. Tested the old one. Just the same as the new one in the pan. Went out for an hour run and took ages to warm and only reached 70*c as with the original and first new one.

I will do more tests tomorrow and try disconnecting the fans.

If they are viscous fans then maybe the viscous coupling is faulty and sticking the fans so the blades are always rotating :Confused:

In the past when I've has a viscous fan seize up, in other words it works full on all the time - hot or cold, then you get a lot of fan noise when you rev.
Normally the fan will semi idle when cold and only begin to lock up and spin with the engine when it gets warm or hot.
Have a listen

Thanks. I still haven't done much testing of the radiator temp to see if it gets warm too early, because a) it's hard to get to any part of it and b) it is usually raining or dark when I drive.

There is no viscous fan on this model, just two electric ones which run at ~12% duty cycle from the word go. I haven't yet found if this is normal, but for comparison, I read on an A4 forum that it is normal for one to run all the time unless outside temp is under 5˚C.

New genuine temp sensor fitted last night, but still never got over 75˚C in over an hour of urban driving in sport mode.

Just a guess- maybe it is getting twisted/squeezed when fitted in housing which restricts free movement?

Also once the thermostat I tested in boiled water seamed to work correctly and I only noticed that it is not closing properly later. It was hard to see because of the shape but it always stuck little open about 1.5mm but what are the chances of it happening with 2 new thermostats.