200 tdi overheating

[mattyme]

ive just recently bought another disco. when i got it, it ran at 1/4 on the temp guage... i know this to be too cold so i changed the thermostat and it runs at halfway now.
driving round town its fine,
out green laning its fine,
go on the motorway or a long run uphill and it starts to overheat. on the motorway i had the heat in the cab turned off and the vents open and it ran over 3/4 on the guage and the warning light flashed. when i put the heat back on in the cab the temp came down again.
its not lost any coolant at all and theres no contamination that i can see in the water or the oil.
i personally think its the head gasket but im troubled because theres no loss of power and no contamination.
could the radiator be clogged with silt or could the problem lie within the incab heating system as it cooled down when i put the heat back on?
i would be very grateful if anyone has experienced similar problems or can point me in the right direction to sorting mine.

many thanks mattyme  ](*,) ](*,) ](*,)

[Range Rover Blues]

If the temp is staying down around town then I'd be thinking duff radiator, it can't cope with the extra heat when the engine is working hard.  Ouras stays just inside the nroaml, say 1/3 unless you drive hard then it climbs to halfway, after around 20 minutes of trundling around it falls back to 1/3rd.  New rad and pump on that one so I'm happy that it works ok.

[lee celtic]

If the temp came down when you put the heater on then the heater matrix is fine I'd say the thermostat or the rad. Take the grill off and get a mate to put a torch behind the rad near the bottom if you cant see light through it theres a problem , I checked mine and the bottom half was blocked with mud and flies and grass  :'( looked ok from the front and back but there are three layers in a disco rad and the middle one was blocked.

[crazymac]

By turning the heat back into the car, you have started the water circulating much more which brings the temp down, thats standard practice for when you are heading towards overheat. I've used the tactic on the motorway before now to get to a safe place or services.

Regarding the other bit, i'd say its down to the rad, disconnect and flush it through properly as a start.