Don't know how much different the 85 is from my 83 but a thing or two I found while trying to diag that same situation with minelast year. The diaphragm above & to the right of the accelerator works the flap that sends air to the vents. During my testing with a hand held vacuum pump on that diaphragm, I was able to keep it open by pulling vacuum and plugging the line to hold the vacuum. My problem ultimately turned out to be electrical - the switch-over valves on the right side of the console.

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Sharing my partner's 2012 Forte 5dr SX til I find my next 123 or 126..
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Do I miss being a service advisor ???