well, I thought it was a thermostat back in the fall making it do it, and it did need replacing, but it's still at it. 83 300dt with climate control. once engine warms up and idles, it blows hot when turned up. But once I get up to highway speeds, it quickly cools off down to barely warm. I'm guessing some kind of vacuum related bypass valve under the hood somewhere? I'm used to levers and valves to control heated coolant.

Any suggestions??

 

that seemed to work this afternoon, but this evening, I noticed it doing it again. I will be interstate traveling tomorrow, so I will be able to get a better idea if this worked. I'm guessing this is just a by-pass servo??

 

Remove the monovalve 4 screws on its lid, then pull up on the lid. You will probably find a torn rubber diaphram within the monovalve. Buy a repair kit to fix it. Pay attention to how the washers and spacers stack on the valve when reassembled. Do this, engine off, engine cold!!.

 

Did you find a torn diaphram inside the MONO valve?