My A/C vents change position under normal acceleration. Anyone have a clue where the problem might be or how to find it &/or fix it?
My gas mileage will be 15mpg with one tank full and drop to 12mpg on the next tank.This happens at random, of course!

 

I duno, but take a look at the gizmo next to the passanger side hood hinge. It has a vacuum line going to it.

I forget what it's called but I believe it has something to do with the control of air for the cab...

 

The air control (vent, floor, defrost...) is controlled by vacuum. If it switches to coming out of the defrosters on you then you most likely have a nice vacuum leak.
Cheers,
Popa Tim

 

ps - defrost is the default (no vacuum) setting by the way, which it why it changes on you

 

The following is a copy of an earlier post of mine. This issue is a recurring problem evidently. Vacuum, Vacuum, Vacuum. Sorry about the length but this worked for me.

Mine did the same thing. First I took someone's suggestion and bought a tester for under $30 at A. Z. called a MihtyVac. I wish I had bought it years ago. It will pump a vacuum like using a caulking gun and has a guage. Start at a funny little thing between the A/C drier and the engine. It looks like a Tee with a little mushroom on it. Three hoses go to it. On my '94 Bronco 5.8 one little hose goes from that little check valve assy. into the vacuum reservoir that is moulded into the side of the evaporator box by the transmission dip stick. That hose had a hole in it so I cut out the bad spot and slipped the ends into a small hose to patch it. It takes several pumps to test the reservoir. Next test the little black hose from the check valve going into the firewall into all your A/C control *****. Pull a vacuum to 20 in. and see if it holds a minute. Then change a **** setting inside and repeat the test for ALL A/C and heat settings. That will test all the hoses and vacuum actuators (dash pots) to all the doors. Mine were fine. Finally I found that little check valve was not holding vacuum. The purpose is to hold the vacuum to the A/C actuators and reservoir up (say around 20 inches) even when your engine gets under load vacuum may drop to (say 5 inches or less of vacuum). If the check valve is not working it may let your panel air go to the default defroster and heater positions in the absence of good vacuum. That check valve assy holds the vacuum high for the controls. It only cost $11 at the Ford dealer. Now it will freeze me out no matter where or how I drive. You can also check your driving vaccuum with the tester. The little valve assy. tested fine until I simulated driving conditions. Here is how I found the problem...On my '94 Bronco there is a hose connector inside the cab mounted on the right of the heater core box. Pull it apart and connect the vacuum tester to the black hose going to the firewall which is the same hose I used tested under the hood. Go up a long hill and glance over at the vacuum guage on the floor. The vacuum should stay high if the little check valve is working . I guess you could also lock the brakes, put it in Drive and put a heavy load on the engine and instead of driving it to test for vacuum drop.

Then on the subject of fuel milage, that could be many things. My 5.8 runs much, much better after I used the MihtyVac to test the charcaol cannister selenoid valve to see if it is stuck open with wires disconnected. It was so I replaced it. It is next to the EGR valve and mouted with a cable tie. Wires go in the top, two vacuum lines out the bottom.

 

Rescued once again! Great info in all responses!!!! LOOOVVVVEEEEE this site!!!!!!
Gonna work on this over the week end. Thanks for the help.

 

b4hntn had some great advice! Although, I strongly caution anyone that wishes to pull the vacuum line off the vacuum reservoir attached to the side of the blower box. If you break the connector off the box, it is almost impossible to fix! You would have to buy a new reservoir and mount it on the firewall or someplace convenient. All vacuum lines in these trucks are either stuck on their ports, brittle, or dry-rotted (in the case of older trucks). Handling vac lines is not for techs with big clumsy hands!! Be careful, don't compound your problems.

Another good test for the two vacuum reservoirs in the engine bay is to simply pulll the lines off the ports immediately after the truck has been turned off. You should hear about three seconds of hissing from the reservoir as it equalizes with the atmosphere.

 

If you're trying to find a vacuum leak, you can use a small propane torch cylinder, turn it on (not wide open thou) and with the engine running, go around all the vacuum connections with the tip of the torch and listen for changes in the idle speed. If there is a leak, the propane will be sucked into the vacuum line and into the engine, that will cause the idle rpm to change...and when it does...there's your leak.

 

Success!!!! Worked on this problem this weekend using info ya'll provided.Bought the vac tool and did some trouble snooting. Found the little 'mushroom looking thing in the location near the evaporator. Mine looks more like a wheel with an axel thru it with a 't' for two lines on one side! The problem was a white vac line (1/8" in dia.) going to the fresh air vent control under the hood, on top of the heater box, on the right side next to the firewall. Severly heat rotted and brittle! Be very carsful! Gonna run my truck for two tanks of fuel, to let it reset it self, then check the mileage.
As I said, THIS IS A GREAT SITE. Again, thanks for the help!!!!!

 

The heating and cooling forum has quite a few posts about that lousy white line!!! I bet you notice a difference in the heating and cooling of the cab too. I believe that line helps control the blend air door.