On a '78 truck is it possible for the water (heater) valve to go beyond closed and actually open up again?
The cable popped off mine and I thought I had closed it by hand but the inside of the truck was still overly warm compared to outside air. Yes, I had the fresh air door in the kick panel open.
Which way is closed? All the way forward or all the way back toward the firewall? I hate working on this type of problem with only one set of eyes.

When the cable is Pushed out the knob is to the left which is the cool side. So with that said the cable should be pushing it closed. I cant really picture the valve off hand. Hope that helps

You may need to replace it...I had that issue with my 76, it eventually wore a hole thru the "closed" portion of the valve, and it wouldn't completely close after that. New valve (less than $10 at NAPA) fixed it. I *think* it's closed when the top is all the way AWAY from the firewall, and OPEN when it's pointed close to the firewall. But, I'm not looking at my 76 right now, so...? You should be able to tell full open from part open by temp pretty easily. Good luck! If it were me, I'd probably just pull it off and see...

JS

You guys are right...closed when the arm on the valve is forward. My cable must've slid in the retainer at the valve too because it wasn't traveling far enough to close the valve all the way.

Thanks!