Ok so a 2001 disco and this happens.
When outside temp is bellow 70 I have no engine temp issues but
When it's above 70 then engine temp can get almost if not into the red.
By running the heat on high temp stabilizes towards the middle of the temp gauge like it always has been.
I have added antifreeze mixed to the resivour when cold but it is always at the right level.
Anyone have any clues where too look?
Thanks

 

is condenser fan coming on? that would be the fan in front of the radiator.

 

That's the electric fan in the front just behind the grill.
I will double check it then

 

the manual gives you the exact lay out, but that fan should come on anytime the truck hit 210 "i" believe.
It should also come on if the AC is on and the OAT is greater than 80 or so. the rave lays it all out im working from memory

 

Even if it working , check if its up to speed , because it not has to work..but spin at the max rpm to pull air in ..