A few weeks ago I replaced the thermostat in our L320 as the temp gauge showed that the engine was only getting to 1/3 hot.
The last few days I noticed that the viscous fan has been running after only a few miles / minutes of driving. The gauge shows the temp go up to centre then drops back to 1/4. It never goes higher than 1/2. If the engine is stopped then restarted a few minutes later the fan is not on and all is OK.
Could my new thermostat be toast after only a few weeks?
Why would the fan run if the motor is not hot (unless its stuck).
What do you think?

Ideally a fault code reader will help diagnose your issue here, but this could well be a Engine Coolant Temp. Sender fault.
Was the engine showing signs of running cold, or did you just change the thermostat thinking it was this?

The viscous fan can also has electronics in it's control, so just because it's on/off doesn't mean you have a low/high temp. (if that makes sense?)
The ECT can give all sorts of symptoms, incl. rough running, as the ECU's can use this for fuelling also. They can fail in a non-linear way.

Changing is an easy enough Job, and they're relatively cheap. Not sure what engine you have tho'. 2011 MY D4 HSE
Previously 2001 D2 TD5, 1996 D1 300Tdi, 1985 90 2.2 petrol.
Bikes! KTM, BMW, British, Classics and others.


Last edited by Bardley on Mon Dec 03 2018 7:56pm. Edited 1 time in total

Engine is the 4.2 supercharged 2005. I swapped the thermostat cause the gauge was showing a cooler engine and the old one was all split inside anyway.
I have a Foxwell scanner so I'll check it with that.
The car has just had a new alternator too and the fan had to come off for that so I wonder if that has anything to do with it.

Check coolant temp. But bear in mind that the reading will be from the ECT.. and that may be reading erratically, so showing Abnormal Temp. on the gauge on the dash.
Also, and I have no firm knowledge of this, the ECU may control the dash temp. gauge.
This is inherently a problem as any overheating issues are usually fed back to the driver too late.
D2's are known for this.
If you're handy with a multimeter, there are readings available for the resistance of the sensor at various temps. Most people
Just replace the sensor tho'. 2011 MY D4 HSE
Previously 2001 D2 TD5, 1996 D1 300Tdi, 1985 90 2.2 petrol.
Bikes! KTM, BMW, British, Classics and others.


Last edited by Bardley on Mon Dec 03 2018 7:57pm. Edited 1 time in total

Where on the engine is the sensor located?

Will have a look... should be a manual online. 2011 MY D4 HSE
Previously 2001 D2 TD5, 1996 D1 300Tdi, 1985 90 2.2 petrol.
Bikes! KTM, BMW, British, Classics and others.

And anyway - I just went out to put the code reader on it.. I started it up and it got up to temp (on the gauge) and it ran ok. No fan coming on and gauge stayed steady.
However, the Foxwell scanner wouldn't fire up when plugged in.. I connected it to my X5 and it works. For some reason the RR is not sending power / signal to the scanner. Odd. If anyone can explain that please do so.
I think I will just replace the cts to be on the safe side

It will be air locked, did you follow the bleed procedure after changing the stat ?

Sorted. Although not without agro.. As ours has lpg some of the pipes were in the way of the sensor and when I lifted the retaining clip it sprung off and dissapeared into the engine bay. It took a rather long time to locate it.. As for the obd port, the fuse was popped.

How can I tell if its air locked?
This morning the fan came on again after 10 min and the temp gauge went from 1/2 to 1/3 as before.
Ill put the reader on it later but it seems like the new cts has packed up already..