Thermostat issues or temp sender?

I have an Audi A6 with the 1.9 TDI-PD (AJM) engine. I've finally decided to address the issue of my temp gauge. Basically the temp gauge starts to climb after about 3-4 miles of driving, less if its motorway.

The gauge gets to about 80C tops when sitting at 70mph. However if I go down a hill with little use of the accelerator the temp will drop. If I drive up a hill hard the gauge goes to 90C pointing straight up until the road levels and it will start to fall again.

The heater blows hot air fine, however I'm not sure if the car has an auxillary heater or not.

I have bought a new thermostat, housing, G12 plus plus, and de-ionized water to replace the thermostat but am now wondering if it may just be the temp sensor. What does everyone think?

Also maybe related or not, the car is a quite lumpy when started below 0C and there is a big puff of white smoke. Does this sound like glow plugs or could a faulty sensor not be turning them on. The light stays on for about 2-3 seconds now it is 1C out. I've never had the glow warning light come on.

Thanks
Will

Strong vote for the thermostat

'Sounds like a classic thermostat case to me.
I wouldn't relate any of the other behavior to this, though.

I agree. I have changed thermostats in Canadian cars for 30 years and that is how they get soft. They should last 5 years but they can start changing in as little as 2 years.

When you take out the old thermostat, put it in a pot of water with the new thermostat and bring the pot to a boil. Then let the pot cool down naturally.

You should find both the thermostats will be fully open in boiling water, but as the water cools down, the new thermostat will shut first. The second will shut much later and will shut more slowly.

If that is the case and you install the new thermostat, you should find not only the motor operates at the correct temperature, but the car will get their sooner and retain it better.

Jack