Please can anyone explain why at a stead 2000 RPM the temperature stays rock solid with the needle in the middle of the guage but at around a steady 2300 the needle quickly moves towards the red.

Is it, perhaps, that the second fan is not kicking in?

Thanks in advance for any helpful comments.

Many possibilities so you need to do some diagnoses.

1) Is the coolant at the correct level in the expansion tank when cold?

2) Does this level fall when hot a) at 2000 rpm and b) sustained 2500 rpm with a significantly hot engine? 

 

 

Thanks for your quick and helpful reply, pikpilot

To point (1), Yes.

In answer to your second point I think the level probably does fall.

 

3 hours ago, FlyByMe said:

Thanks for your quick and helpful reply, pikpilot

To point (1), Yes.

In answer to your second point I think the level probably does fall.

 

How far down the expansion tank? 

Don't know for certain but, not so far as to trigger the low water sensor.