Hot air all the time

[mach1mustang351]

Team.  I have hot air coming through my vents at all time.  I suspect a bad heater control valve. Anyone else have this problem??

[mechanic80]

I have hot air at the vents too, (if the mode door isn't closed) but I replaced the heater valve with a a non-a/c valve because of the cost.  The valves  are physically identical (to all outward appearances) and the diaphragm does stop the flow of coolant.  I think it's the fact that hot coolant gets to the valve and the rest of the heater and valve gets hot because of, what's the word, convection? The A/C system does not have a charge and I'd like to get it working again, but the cost is prohibitive for now. 

[Eagleearl]

The shutoff valve should have vacuum to it when the control is on A/C. If it does and the valve is not shutting off there is something wrong with the valve. It may be stuck and you can move it back and forth to loosen it up or the diaphragm may be leaking. If you have the control lever on cool the flapper door in the heater core box should shut off the air flow through the heater core and let cool air flow through.

[BenM]

Heater control valves are normally open (they allow water to the heater core) and close with vacuum applied. I have found four styles available. Three are metal, two similar for the six and one for the AMC 150 that has several bends. These three are generally priced at $80-$100.

The forth style is specified for Jeep Cherokees through 1998 and is plastic, after that Chrysler simply dropped the part and allowed the heater core to remain in the loop at all times. The plastic part does age faster, and there are common complaints of it weeping from the valve, but the big advantage is that it is available for under $20 at any parts store.

The Cherokee part has four 5/8 nipples, I think that the metal ones have 3/4 on one side.

[Still Pat]

Same problem, I can't get down under the dash to check the door/flap is functioning properly (body won't do that). The lever slides okay from A/C to heat & the temp. lever slides okay, just no change in the temp. and it DOES put out heat (does it ever!). Afraid to disconnect the heater core because if I do, with my luck, it'll start leaking in the winter when I hook it back up.

[trucker79005]

Can you manually move the lever on your heater control valve under the hood to see if that stops the hot air?

[Still Pat]

I can try next time I drive it.

[Still Pat]

I can try next time I drive it.

No lever. Strictly vacuum. I looked.

[mudkicker715]

Above the passenger feet is the heat cold blend door thing. Maybe that cable is slipping on the clamp, and stuck on a hot blend for your uncomfort?

[Still Pat]

If my body would move that way!  Not as young as I used to be. I was figuring something behind/under the dash & I just can't do it anymore. 

[amarshall]

I can try next time I drive it.

No lever. Strictly vacuum. I looked.

Sorry if this is a repeat, but I just went through this with my heater valve under the hood.  It's normally open, so hot coolant will flow into the cabin and circulate through the heater core.  When vacuum is sent to the valve, it should close freely - and the vacuum mechanism moves a very small lever on the outside.

Try connecting the heater valve directly to a vacuum source - bypass the existing vacuum connection which goes into the cabin to the HVAC control head.  This should close the valve and prevent the heater core from warming.  If this doesn't stop the heat, the valve is bad.  All that can be done on top of the engine.

If you find that the valve works (the heat stops when it has vacuum) then the problem must be under the dash.  Either the HVAC control head has a vacuum leak or a bad switch, or the blend door is broken.

[Still Pat]

I can try that next time. I'll double check for that lever, all I saw were the heater hose & vacuum line.

[trucker79005]

I can try that next time. I'll double check for that lever, all I saw were the heater hose & vacuum line.

For some reason I think one of mine has a lever and the other didn't so don't sweat it. What he said on applying vacuum directly to the valve first to see if it stops the air.  Mine had both a faulty valve and had issue behind the heater controls where the vacuum line plugs in AND the cable had disconnected from the peg on top of the heater controls. None of them terrible hard to fix but takes a minute to get the center piece out to access the controls.