Another Disco Thermostat/Heater Question

[Rineheitzgabot]

I am sure all of you experienced rover fans have heard your share of heater/thermostat problems with Discos in your time, but I have another one.

In my '02 Disco, Petrol, no heat will be generated out of the heater unless the RPM's are increased to at least 1800. If I sit in park ir neutral and rev the engine up to 1800+ when the engine is at normal operating temperature, copious amounts of heat will pour out the ducts. If it sits at idle speed, the air that comes out of the ducts is cold--burrr. Is this a thermostat issue?

My wife, or uhm I, would like some assistance if you have any answers. :)

[rvrsrvc]

So, the increase in rpm moves a larger volume of coolant through the system. Is the system full? I use a device that creates a vacuum on the cooling system, and it fills through the device where it attaches to the expansion tank. The advantage of this is that the system fills from the bottom up leaving any air at the expansion tank.
I'd take a guess that the coolant level is low or air bound.