Good afternoon I'm looking for some advice on a strange issue with the cab heater of my D22 2.5 diesel 4x4 truck, a few weeks ago the heater stopped blowing any hot air, the temperature of the engine was normal, with this I went ahead and got stuck in swapping out the thermostat and water pump for new, flushing the rad and heater matrix which all seem to run through ok, but this has not solved the issue I only get heat is i rev the engine above 3000 to 4000 then it just cools back down and blows cold air again? the top hose seems to pressurise but no fluid comes out the overflow at the radiator cap so cant be that high? Im now at a loss.. please help.