(The site won't let me post this under "Interior/Trim", so didn't know where else to put it). I recently removed/cleaned/replaced all under-dash HVAC parts in my car that had been sitting for years. When I slide my heater control to "heat", I get full heat coming from vents or heater or defrost when at idle, but it won't stay hot when driving at highway speeds. I'm thinking the door is not staying in the correct position. Is it possible that air is coming in & blowing the door open at highway speeds? It seems to do it equally at all blower motor speeds. I first thought vacuum leak but isn't the door mechanically controlled? I'd like some opinions on what to look for before I start removing the under dash parts. Thanks!

 

Check the vacuum lines at the ball under the hood. It sounds like you may have a leak somewhere.

 

And no, it's all vacuum operated with diaphragms to operate the doors.

 

thanks Randy - I'll go chasing vacuum lines this weekend & tear into it if needed.

 

OK, I was thinking of the later vacuum controls. These are partially cable operated.

 

Vac to heater valve?

 

OK...lol....that set is for non-AC heater controls.

Here is the AC stuff...

 

That's what I was thinking (your pictures are better than my service manual pictures!). I think I'll check the vacuum lines I can get to easily first, then go after that cable adjustment & all other vacuum hoses.

 

First, I ASSUME you have A/C, since you didn't say. Second, if the system works at high vacuum at idle but not when the throttle is opened (and vacuum drops) then you obviously have a problem with the vacuum storage ball and/or the check valve that retains vacuum in it. You might also have a leak somewhere that's small enough to allow the system to work at high vacuum conditions but not at medium.

Most likely the problem is the vacuum operated heater control valve. this is a normally closed valve that opens with vacuum. If vacuum is inadequate to hold it open, flow of hot water to the heater core is blocked.

 

These are from my service manual and I took the pics with my phone.

 

thanks Joe - yes, it's an AC car & you're describing the problem exactly.

 

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