I've had an ongoing problem with ice-cold air blowing from my defrost when it should be hot. Fortunately, I'd say 90% of the time it blows hot air like it should, though it used to only blow heat about 40% of the time, so it has improved. Here's a quick summary of what has happened so far:

- I was losing coolant, so I thought maybe it wasn't heating due to the lack of enough coolant in the system.

- I had a coolant leak from the thermostat housing. I replaced the thermostat housing. Leak stopped. Things got better, but it would still blow cold air, fairly frequently, after running it for awhile.

- I noticed that if I put the climate control to "Hi", as opposed to '90'f or less on "auto", I would always get heat. Sometimes I could also get the heat to come back by shutting off the climate control panel, waiting a few seconds, and then turning it back on.

- I thought maybe I had air bubbles that hadn't worked out of the coolant circuit, so I made sure I purged it and topped it. It got better.

- Today, I had it on auto at about 80f, and I noticed that my feet were getting warm air, but the defrost was cold a/c air.

Here are my thoughts about what it could be, and I'm hoping I could get some feedback:

1 - The cold-air bypass flap could be getting stuck (although the climate control self-test doesn't indicate so). This might explain why I sometimes get foot heat but ice-cold defrost.

2 - The heater valve could be getting stuck closed while in 'auto' mode and cycling, but forced open when I have it manually set to 'hi'.

3 - The heater pump is failing on occasion.

 

The fact that you get heat if you switch to HI means that the fault lies with the temp control. The system compares actual cabin temperature to the set one when you set a temp up to 90, but overrides that control loop in HI mode. If it thinks the cabin is more than 90F all the time, due to a faulty sensor, this could be the problem. My first port of call would be to change the cabin temp sensor (or motorised aspirator as they call it)

 

I had an identical problem on my XK8. Cured it by getting lucky. I pushed the outside temp button one day and the readout said it was 136 degrees outside. if i put it on HI it would provided heat but in auto only got airconditioning. I would look into the ambient air sensor out around the front bumper.

 

Thanks, Mark, I'll check it out. Does this still hold up to the symptom that I get hot air blowing at my feet while cold A/C air is coming out of the defrost?

@avern1: Thanks for the suggestion. The outside temp reading seems fine.

 

"I get hot air blowing at my feet while cold A/C air is coming out of the defrost?"

Does this happen even in HI?

 

That's a good question - I'll have to double check, but I believe it does not happen in HI. I wouldn't expect it to, since the AC should be disabled while in HI, meaning at the worst I would be getting cooler air, but not refrigerated air, which is what I believe I was getting.

Do you happen to know if the thermistor for the cabin temp is easily monitored either with a standard OBD2 or multi-meter?

Is the water valve even a possibility? As I understand it, the valve has a duty cycle while in auto mode, and is fully open while in HI. My thought was that maybe it is occasionally sticking closed while in duty cycle iterations.

I'm, of course, avoiding the potential discussion about the A/CCM possibly being the culprit. :-)