My Nissan navara d40 2007 Truck is blowing cold air when the heater is on hot and the engine temperature gauge is going up really high if the revs exceed 1000 and when I decrease under 1000 revs it goes down to normal again and as long as I am crawing under 1000 it stays in the normal postion but no warning lights have come on Any diagnostic help is welcomed. Thanks in advance.