Actually, this is a real common problem with an easy fix. Your mode doors are operated by vacuum motors. You're losing engine vacuum when it's under load or when accelerating. There can also be a leak in the system that prevents the controls from operating correctly. If that were to happen, these systems are designed to default to the "defrost" position. Your feet might freeze, but your windshield will be clear. It's designed that way for safety.
At the mileage you listed it's normal for the engine to not develop as much vacuum as it did before. The first thing to do is switch to "defrost" with the engine idling. Stop the engine, then try to select a different mode. You should hear some squeaking or rubbing as the doors start to move. If you do, there's little chance there's a leak in the vacuum hose. If you don't hear anything, we might still have to check that hose for a leak later, but lets go with the fix first and see if that solves the problem.
My red arrow in this drawing is pointing to the check valve in the vacuum hose that runs into the passenger compartment and runs the heater controls. It's on the firewall under the wipers. That valve is about the size of a stack of three nickels. The dealer's parts department has a replacement check valve that is roughly the diameter of a 50 cent piece, and about an inch and a half long. This one includes a small vacuum storage canister. It can be used to solve this problem on other brands of vehicles.
All you have to do is describe the symptoms to the people in the parts department, like you did here, and they'll know just which part you need. I don't remember if there's an arrow on it to tell you which way to attach the two hoses. If the heater controls don't work at all with the new valve, just switch it around, then reattach the hoses. There's a good chance you can find this valve in a salvage yard too.
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Wednesday, September 15th, 2021 AT 6:05 PM
