Heater temps fluctuating

[ThreeQuarter]

The temperature of the air blowing from dash, floor, and defogger vents fluctuates with engine/vehicle speed. When I am moving down the highway, the heater could bake a turkey. But if I stop and sit at a traffic light, the temps get kinda coolish. When I'm in town I am having to adjust the heater temp more often than my radio station. Temp control adjusted to full-hot at a stop light equals 50% Hot/Cold at highway speeds. The engine temp gauge is steady throughout all of this, and the blown air temps fluctuate faster than I would think coolant temp could rise & fall, anyway. I've never had a vehicle that did this, and this one has done it ever since I bought it in '04. Any guesses?

My first thought is a restricted heater core or hose, so much so that the coolant pressures at idle don't force much through the core, but higher pressures at highway rpm do. However, I installed a high-capacity cooling system (pump, dual-stat crossover, etc.) a couple of years ago and that didn't seem to change anything in this regard. If core restriction is the problem, should the high-capacity system have an effect?

I'm stumped. Your input is appreciated.

[Hubert]

Did you purge the air out of the crossover? Maybe an air bubble stuck in the heater core?

I think the coolant circulates constantly and the temp is controlled by changing the air flow. So I'd guess a bad actuation motor or something wrong with a vent passage.

[ThreeQuarter]

After I installed my cooling system upgrade I had the system flushed by a radiator shop (best in town, in my experience) and they did purge the system. I've checked several times since, and there's never any air under the bleed screw on the crossover.

Yeah, the coolant should be circulating constantly, and the temp should only change by mixing cold & hot air, right? Can you explain how a bad actuator motor can cause this? Or how a vent passage could make it behave this way? At first I thought I had a vacuum leak causing one of the dampers to slowly close at idle, but then I remembered that these pickups don't use vacuum actuators...so I'm really stumped!

[Robyn]

The water pump does slow down at low engine speeds but there should still be enough hot water to keep the cab warm.

My best guess is that the temp control or the servo motor is not adjusting the blend air door to full hot but instead a partial setting that is not quite enough at low speeds.
The other thing could be that the heater core is plugged up to some extent and not allowing enough hot water to flow through to give a nominal amount of heat.

Robyn