If the temperature control valve, which controls the water flow through the heater core is stuck or partially clogged it can cause that. If the flow valve is O.K. then your water pump isn't circulating enough water at engine idle. With low water flow the cold air overpowers what heat the core receives from the water flowing through it. When you speed the engine up the increased speed of the engine causes the water pump to move more hot water through the heater core thereby providing enough heat transfer so that the incoming air is heated enough to make you comfortable. The vanes, on your water pump could be deteriorated from corrosion over the years. A new pump should cure the problem.I have an 89 GMC Pickup, the heat blows cool. Electronic controls, new heater core, thermostat and water pump.
Check for vacum leak at diaphragm for vent control. Bleed coolant system of air.
Its a Vaccum leak to your Temp selector. When you are at idle (lower vaccum) it will think you want a low temp. When you step on the gas and raise idle you vaccum will go up and so will the temp.
Apparently it happens to a lot of Chevys. I have had it happen my dad had it happen and other guy a few minutes ago asked about it
It sound like the heater core is plugged, I had this same problem and after replacing all you have, I found the heater core was plugged. Turn on the heat with the hood open and see how long the two heater hoses stay hot if one gets colder than the other after a few minuets then replace the core. Good Luck and God Bless
Blend door issue-in the heater box. Common problem.
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