2.5TDi Temp gauge has mind of its' own!!

[tonycaterham123]

Hi

Temp gauge acting up. When start up car warms up nicely to 90c and stays there for anything from a few minutes to half an hour or more, then steadly drops to 60 stays there for a varying length of time several minutes then rises back up again may hand around 75 for a while before going back to 90, then repeats process. Car runs fine and pulls strongly and has been doing this for 500 miles or more. This happens on short and long trips.


Is is probably sensor and is it an easy job?

The car has done 79800 miles so due for cambelt change soon.

 

[Nessy]

I would guess that it is coolant temp sensor on the bulkhead as this seems a very common failure on A4's (my own car had this replaced last year).
The one thing I would say is does the heat output from the heating system seem acceptable?
I ask as I acquired in the summer an 80 TDI a few years ago; when the autumn came I couldn't get over how bad the warm air output was from the heater, couldn't understand how a German car could have such crap heating!
It turned out that the 75 degrees reading on the gauge (and which it had been doing all summer!) wasn't right, the thermostat had failed and was causing car to run too cool.
A new thermostat caused the gauge to read 90 degrees after that, with a heater you could roast with, lol.
Its doubtful a duff thermostat would cause the symptoms you describe but its worth thinking about........

 

[AndyMac]

Probably thermostat. I've had both temp sensor and thermostat problems at different times. Temp sensor issue was pretty digital. It either read correctly or not at all. Thermostat problem was exactly as you describe. It would only creep up to 90 when stuck in traffic. Oil temp was low as well. Get the thermostat done when they do the cambelt & water pump.

 

[kris_tdi]

most probably a sensor. mine sits at 90 degrees pretty much all the time once warmed up.

 

[tonycaterham123]

The car is coming up to 80,000 mile service so as having cambelt change might as well get thermostat and sensor replaced at the same time. The bill is going to be that big I'll not notice these on top of everything else!!

 

[graham A4]

have the same problem myself changes the thermostat which fixed problem for a while the changed the seal on the coolant temp sensor but its back to its old tricks so il change the sensor itself which should resolve the problem