Derek1

I have 2002 Audi A6 C5 2.5 TDI 132 kWh. Recently my coolant temperature started rising to 100 °C and sometimes even beyond that. While idling, it reaches 100 °C in 10-15 minutes. However I'm not sure if it's a bad temperature sensor or the coolant really gets that hot. Also when I drive it and use climate control, the car gets warm inside, but now I've tried to put it on a maximum heat and the air was only warm instead of hot, temperature was at 100 °C at the time. Could it be a bad thermostat or air in the coolant? When the coolant was at 100+ °C I've carefully opened coolant expansion tank cap and while I could hear pressure release, the coolant wasn't boiling at all. Another thing that seemed odd was when car cooled down for sometime and I've turned on the ignition, oil temperature gauge instantly rose to its optimal temperature, but then slowly, over the next ~3 seconds lowered to its current temperature. I don't think that I've seen this happen before, could that somehow be related? How should I tackle this problem?

georgeb944

Verify the coolant temperature with an infrared heat gun on the upper coolant hose to the radiator. If the coolant is not really that hot, then you will want to replace your coolant temp sensor. If it is that hot, probably time for a thermostat.

It might be something like a bad ground also.

If you have VCDS, watch the engine temps in the Measuring Blocks... If you don't, get VCDS.

Derek1

This hose? If so, then I've held it with my hand and it was warm to the touch, definitely not hot. Unfortunately I don't have a VCDS.

Could a bad coolant temperature sensor cause heater to not blow hot air when set to the maximum?

Derek1

After a night, I've got into car and turned on ignition, the coolant temperature gauge jumped up to 90.

It seems like it's a coolant temperature sensor that's faulty.