The heat was barely adequate last weekend coming back from Chicago. The coolant level is ok, temp gauge at 12:00, heater valve moves like it should, vacuum checkvalve works, the lower hose is hot while the upper one is warm (fan on high, temp switch on max). After a few minutes of highway driving, with temp switch set to max and blower on high, I noticed cool air coming from 2 dash vents that were open. I stopped to get gas, shut the car off, then restarted it and no air came from the upper vents (other settings the same). Is there some way that air can infiltrate without passing the heater core? Anything else I should check? Thanks.
Marlin

Marlin,

Put the temp controller in manual and moved the temp selector to max temp.

The control valve that controls the water into the heater core is behind the engine and works by vacuum.


Please check this first and let us know!!!

The vent switch is on manual and set to the floor vent. The temp is set to max. The coolant control valve in the engine bay is connected and functioning correctly (pulls toward valve body when temp set to cold, and away from valve body when set to hot).
Marlin

If you follow the vacuum from the valve back to entrance into the cabin you will find 2 other vacuum lines with one way valves. These lines brings vacuum to bellows that control the air flow to the different positions. Change the position from the bottom damper to vent straight like AC on and see what the temp is know.

The other possibilities are the thermostat and a temperature sensor reading bad.

I just change the sensor in the 960 because of a weird idle problem.

If you are in Chicago drop me an E-mail: humbertoflores63@hotmail.com

HR, thanks for the info. I think the thermostat is ok because the temp gauge reads ok and the lower heater hose is hot to the touch. When I switch to the upper vents, hot air comes out initially, but cools down in much the same way as the cool air coming out of those same vents when the switch is set to the floor vents and the car has been driven for a while.
Marlin

you need to check those one way valves that bring vacuum to the bellows inside the climate control.

The valves are outside by the passenger side or you can follow the vacuum
line from the heater valve back into car.

I am not sure to post a schematic picture of this bellows. Sent me an E-mail if you like the schematic