My landrover TD5 as a problem,i was told by the people i purchased it from 4 months ago ,(i'm now one month out of the warrenty ) that after i told them my problems which are as follows ,if i'm idleing the heaters blow out cold air but as soon as depress the throtle up to 1.500 / 2.000 rev's the heaters get hot, i first changed the thermostate,then a new water pump which solved the problem and the heaters seemed to work fine for a week or two but he same problem as returned with slightest amount of coolant lose, their answer to this is that its the headgasket,they say its common for the TD5 to react this when the gasket as gone , but theres no sign of oil in warer or water in oil no lose of power no overheating and plenty of power.

 

Not heard of problems on the TD5 before, it is know the engine does run cold compared to older diesel engines, but not heard of head gasket problems been common.

Where are you located in the big wide world? Might give a better understanding of the local weather conditions.

It might be that the valve that lets water into the heater matrix is stuck, or some form of blockage in the pipeline, thus when you rev high and put the system under more pressure, water is actually getting to the heater matrix, when you have low revs the water is not running through the heater.

The slight loss of coolant after the change of thermostat might be the result of an air lock been cleared, have you checked to make sure the cooling system has been bled, and contains no air locks?

The head gasket should be the last thing to check, you should check the above, but also check the radiator for any damage, (it might nto be letting water through it correctly, check the heater matrix has not been damaged.. (if easy to get too, mine is on the defender 90)