ttroyer

I have a 1986 GT Convertible. I'm experiencing issues when the Heater/AC control unit is set to "Vent" or "A/C". I've removed part of the interior, so I can see the door that directs the air. When I set it to a setting that should blow through the vent, it works, until I start to accelerate. As I'm accelerating, I can watch the door slowing close. As soon as I stop, it slowly rises back up, unless I start accelerating again. So, my air is constantly switching from vent to defrost/window, and vice-versa. Has anybody experienced this before? Does anybody have any idea what might be causing this? I'm assuming it's related to the vent door, and not the control unit itself, but I'm open to any suggestions.

randy78045

Just throw all that stuff away... it just adds unnecessary weight anyway...
Seriously, it might be vacuum related..

ttroyer

Sounds logical, but I'm an expert on how the vacuum system works, so I'm wondering a few things, including what to do next?

As I'm accelerating, the door steadily goes down, and steadily moves back up when I stop accelerating. I'm wondering if the vacuum pressure is most likely remaining the same, and it's just not enough to fight the g-force during acceleration, or if the vacuum pressure is actually decreasing for some reason during acceleration.

Does anybody have any insight or suggestions as to what might be a good first step towards fixing this? I've taken a quick look at the vacuum hoses under the hood, and don't see any hoses that seem to be leaking.

mj88stang

i do believe there's a check valve in the vac line that goes from engine vacuum to the hvac box. it may be missing, installed backwards, or just plain bad. i forget exactly where it is at but if you follow the vac line you will find it.

monkeydude3

+1 either that check valve is bad or the lines to it/from it are. vaccum drops when you accelerate

PRO50SC

Check the connection to the vacuum canister, that's usually what causes your symptom. If that's ok, the cannister could be bad(leaking)

ttroyer

Thanks for the input. I didn't find a direct replacement check valve at Advanced Auto, but did find a universal one that I replaced the old one with. I also replaced the line going to it (but have not yet checked the two lines going through the firewall, which I'll do when I get a chance to tear apart the dash). After replacing the check valve and vacuum line, I saw improvement... The air goes through the vents for a longer period of time, and takes less time to return once it's gone. The check valve I got is smaller, and made for GM vehicles, so I'm thinking I'll try a Ford dealer for an OEM part.

As far as the canister, I'll have to check that. I believe that will be under the dash, right? Any hints as to what I'm looking for?

PRO50SC

Actually I think it's in one of the fender wells.

ttroyer

So does the vacuum canister support the whole vacuum system, or just the A/C portion? I'm also having problems with my brakes.. the brake pedal is spongy. I've replaced the master cylinder, the wheel cylinders (one was leaking), and have bled the entire system. I didn't think this was related, but if the vacuum canister is leaking, could this cause poor vacuum pressure for the power brake booster, and be causing both problems?

LS1killin92stang

sounds like, in ur pass. fender, your vacumm reserve canister, there are two, one runs the A.C and stuff and the other runs ur EGR and thermactor air bypass sol. sounds like its bad, or weak, and to answer your question about vacumm under acceleration, theres a point when you hit the gas, that you create a void in the air vacumm, because the engine, all of a sudden has a change in the amount of air its pulling in, so the manifold vacumm drops, this always happens, usually about 5-8hg anytime the rpms of the engine rise more than say 800-1000 rpms rather quickly. So, since your cannister reserve is going bad, when this happens, the vacumm is dropped to the vacumm controls that open and close your vents, it a simple fix, but if you do it, replace both canisters while your in there and the one way check valve. and, no. the brake booster is run off direct manifold vacumm, it has its own seperate line spoungy brakes, are usually air in the lines, you need to bleed them, low vacumm or a a faulty booster would most of the time cause a stiff pedal and feel similar to pressing the break whilethe car is not running. A leaking wheel cylinder needs to bereplaced because if liquid is being able to be leaked out, then air is being sucked in, this is most likly your prob.