dvs_dave

That's why I did further tests as a faulty sensor or module would typically throw a code or give an implausible reading. I have no fault codes, so resorted to more traditional diagnostic methods by monitoring the live data feeds and what effect various actions had on that data. This lead me to my conclusion of a partially stuck thermostat. Do you think it could be something else?

When the engine temp was at 106 degC, guage showed normal and didn't budge even a fraction of an inch. I suspect it has a deadband between 80-110 degC before it starts to move away from 12 o'clock position. I guess it stops people worrying until there is a real problem but it's clear that the gauge is dumbed down and no more useful than one that simply says "cold, normal, hot".

mishar

If that temperature sensor is designed as you suggested it may be that you actually do not have any problem. 106°C is not too hot. Whether fans should run high depends on speed. Thermostat might be fully opened (should at 102°C) but low air circulation won't cool the radiator enough, so fans kick in high. Just contemplating from a distance.

Actually I may hook VCDS and let the engine idle until I get fens running high and see what the temperature will be. Or I should fix a drainage problem at the back of my house? Hmm...

dvs_dave

Hook it up, see what it does. Choose coolant temp and fan1 and fan2 duty cycle measuring blocks and also note the temp gauge position during the process. Try pulling the fan fuses to see at which point the gauge actualky starts to show hot.

My fans were still running at 70% cruising along at 50mph (had the laptop on passenger seat, on a private road - officer ), coupled with very little heat coming off the radiator even though the fans were blowing hard were the two biggest clues for me.

106 degC is on the hot side but still ok and I think I've perhaps just caught the issue sooner than most as there are no other obvious symptoms.

mishar

I would do it right now but I don't have any private roads nearby.

dvs_dave

I had the thermostat replaced (as part of a timing belt job) and the car and fans are now operating normally.

Below is the data log snap shot of the engine temp and the fan duty cycles with the AC off. As you can see VERY different results this time and this is what they should be. This reading was taken with the car fully warmed up and after 5 mins idling in 55 deg ambient temps. The flow of air coming off the radiator was now hot, as it should be.

Address 01: Engine (4E0 910 560 A)

19:44:16
90.0°C Coolant Temperature (G62)
10.2 % Fan 1 Activation Duty Cycle
10.2 % Fan 2 Activation Duty Cycle
Compr.OFF A/C Compressor
0.0 % Fan Request from A/C-System

I tested the old thermostat (trigger temp of 82 degC marked on it) in a pan of boiling water, and also dunking it in cold, and it didn't open or close at all so confirming it had failed.

Anyway, hopefully this will be of use to someone faced with this issue.

mishar

It will. If he finds search function first.