Alan

This points to a vacuum leak - at low engine loads (like idle) you have plenty of intake vacuum available to use (and overwhelm a small leak), but with the throttle open (at load) the engine intake vacuum reduces - and you rely only the vacuum tank - this can deplete quickly if there is a significant leak or if the one way valves are shot.

Seem you have such a vacuum leak. This is probably impacting the comb/bypass flap - however I wouldn't expect this to make very much difference unless there was also a partial water valve leak as well.

Alan

WallyP

You might check to make sure that the black side of the heater valve is closest to the engine. If it is backwards, increased RPM can cause enough coolant pressure to force the valve open, allowing hot coolant into the heater core.

NC928S4

Thanks Alan and you are correct. Once again I had to thank the PO for the wonderful state things were in. I had a bunch of things going and have been fixing them one at a time.

1) I was low on refrigerant because of a leaky schraeder valve($3 fix). Added more refrigerant but ran out before target pressure was reached.

2) The ambient temperature sensor under left front fender was disconnected which was causing the heater control valve to stay open 10-15% when I did a visual. Once I hooked the sensor back up the heater control valve closed completely. This helped with cooling.

3) Roger suggested the paint stirrer trick until I get around to fixing the recirc flap diaphragm. This helped the most especially when driving. I also topped off the refrigerant.

4) I still have a vacuum leak somewhere in the console as I hear a definite hiss at idle. When turning the car off I can hear this hiss slowly decay to nothing.

While I won't get frostbite from the AC, it works sufficiently until I find the time to make the additional repairs. I got diverted to fixing a seat switch which was a RPITA.

Alan

Glad its working a good deal better at least

If the heater control valve wasn't all the way closed it was some kind of vacuum issue.

The electric solenoid valve controlled by the temp sensor loop (via the head unit) is like a switch - it either selects vacuum or no vacuum (actively vents) to the water valve actuator - purely digital. If the water valve was partially open that has to be a vacuum leak after the solenoid valve or a low vacuum source feeding it.

I expect that if the sensor loop was open circuit the HVAC should go into full heating mode (water valve open) - unless the temp selector is fully in the max cold position

Alan