Hi all, 96 Explorer 4.0 w/ manual temp control. I've had a very occasional issue (occasional in part because I live in SoCA and rarely use the heater) with the heat being non-functional. Usually it works fine, but occasionally it will come on only after I play with the heater control knob. However, I haven't been able to establish whether this is a coincidence or not. I also have an issue where on any 70+ degree day, my vents (ie temp control set to vent air from the outside) will start to deliver warm air. Sometimes it gets warm enough that I turn on the a/c just to cool down the vents, and then I have to repeat the process every 10 mins or so. I read through the forum and decided it might be the blend door, so I pulled the actuator but it works fine. The blend doors do make a bit of a "clump" sound when the actuator reaches the end of its transit at the "heat" side. Then I decided to test the heater valve. I turned on the car and had my mom turn the temp control knob from cold to hot, wait 5 seconds, and go back again. I expected to see the metal/plastic arm on the bottom of the valve extend/retract, but nothing happened. The vents delivered cold-cold to blazing-hot air, but I didn't see anything happen to the valve. Finally, I decided to unplug the vacuum line from the heater valve to check for vacuum. No discernible vacuum from my informal test. So my questions: 1. How much vacuum should there be at the heater valve? Should I be able to feel it on my skin? 2. What does the vacuum do? IE, when the temp knob is in the cold position, then the valve ought to close, keeping hot coolant out of the heater core. Does the vacuum *hold the valve closed* or does it *hold it open?* 3. Any possibility this is the cause of my hot duct issue? IE, maybe the heater valve is letting some hot coolant into the heater core all the time, and that's why my vents seem to warm up while driving? I should mention that I've also had a recent issue where the car isn't getting to normal temp...just about 1/3 of the way. It runs fine and I get 20 MPG, but it never seems to get to normal. It has a couple of new hoses and a new radiator cap, and there's no air in the system. Could this be due to the heater valve, maybe keeping things a bit too cool by having an extra radiator in the mix (the heater core)?