My coworker's wife had a concern about her car. He says the car gets up to temp when driving but when at a stop. and has the heat kranked. He says that the heat output will get cold. Is this suppose to happen?
If I understand the question properly it is the heater matrix, the matrix is a little radiadtor in the dash when the heater blower is turned to full it will cool faster than the engine can circulate hot water through it. If the engine is revved up but not moving and the heat returns, no problems but if it doesnt the there could be a blockage in the matrix itself. Check the water level, check if the water is clean (not a rusty brown) and has the system been empty of water and then refilled there could be an air lock in the system. Please correct me if you are just talking about the temp guage.