Can only get air through defrost vent or floor vent

[Ncgomez65]

I have an 86 f250 and I can only get air through my defrost vent or the floor vent, until I get on to the freeway and then the air will start blowing out the normal vents. If any of you can help point me to an area to check that be great. Thx for the help

 

[stumiister]

The HVAC blendoors in our trucks work off the vacuum system of the truck, my truck does the same thing I know I have a vacuum leak at the brake booster, I would check for vacuum leaks, I would probably use propane for this, if the engine idles different than without propane there is your leak, I would start under the hood at the vacuum pump, and follow the lines until you find a defective part, replace the part and continue the search for more leaks.

 

[cpdenton]

I am pretty sure sniffing out leaks with propane would not work on our trucks....since the vacuum is pulled by the vacuum pump and the propane would never enter the combustion chamber. The vacuum has nothing to do with the engine.

Unless there is something I am missing.

To the original poster....it does sound like a vacuum issue. Spend a while looking over the lines and see if there is a obvious leak first.

 

[stumiister]

A big colprete of vacuum leaks is the brake booster, I would listen for a woosh or whistle sound near your feet inside the cab

 

[Ncgomez65]

Thanks for the help.

 

[bike-maker]

On the 80-86 trucks with factory AC, the blend door that switches the air from defrost/vents/floor has a habit of breaking the hinges off. Happened to mine. The factory used plastic hinges that break over time.
I was able to get to it by removing the glove box and dash pad, and I used some small hardware store steel hinges to repair it.
But definitely make sure your getting the correct vacuum signal to that door first; it's kinda hard to get to.

 

[wanabebig]

How did you get the hinge in there? How long of one did you use? I need to do this to my truck while I have the whole dash out since its broke. Before I put the dash out a k in tomorrow, got any type of pictures maybe?

 

[bike-maker]

I used 2 small hinges, only about 2" long each, and a strip of sheet metal to reinforce the whole mess.
I didn't take any pics of it.
There is a pretty good right up on it over on FTE, I'll see if I can track it down...

 

[opusd2]

Whenever I had issues like this, it's usually a vacuum issue. Since it seems to take a while to build vacuum up to the point where it operates the door, I would check the output of the Vacuum pump, the belt quality on it, and maybe put an extra vacuum reservoir with a check valve so it sits opposite the brakes. The brakes would definitely show if they weren't getting much vacuum to assist it and a leak isn't the worst to figure out. Even my 79 Ford cab's HVAC controls require decent vacuum and I had a problem like this once that I narrowed down to a leaking connector.

And the propane test works on gas engines because of how they work, just not our diesels. Besides propane you can use a number of aerosol lubricants and other propellants to check that, but a gauge is so very valuable. On a gasser a permanent mounted one can help you run the vehicle for better gas mileage and operation. And if your 86 has a C6 tranny, the lack of vacuum will cause it to rev more so it is an important part of the vehicle's controls.

 

[The Warden]

On the 80-86 trucks with factory AC, the blend door that switches the air from defrost/vents/floor has a habit of breaking the hinges off. Happened to mine. The factory used plastic hinges that break over time.
I was able to get to it by removing the glove box and dash pad, and I used some small hardware store steel hinges to repair it.
But definitely make sure your getting the correct vacuum signal to that door first; it's kinda hard to get to.

FWIW I had the same problem that bike-maker described...I was able to order a new door assembly from Ford, and I must say that installing it was a cast-iron PITA. If you have a friend with small hands, begging him/her to do the job might not be a bad idea. As mentioned, check the vacuum system first, although generally a vacuum leak will make the HVAC default to the defroster position...

 

[sassyrel]

How did you get the hinge in there? How long of one did you use? I need to do this to my truck while I have the whole dash out since its broke. Before I put the dash out a k in tomorrow, got any type of pictures maybe?

if your dash is broke,,go for it..you can remove the heater box assm..by taking the dash bolts on the right side out,,and puling out on it,,while removing the heater box.. taint easy,,but doable..

 

[MIDNIGHT RIDER]

On the 80-86 trucks with factory AC, the blend door that switches the air from defrost/vents/floor has a habit of breaking the hinges off. Happened to mine. The factory used plastic hinges that break over time.
I was able to get to it by removing the glove box and dash pad, and I used some small hardware store steel hinges to repair it.
But definitely make sure your getting the correct vacuum signal to that door first; it's kinda hard to get to.

I am afraid this is my new problem.

Just all of a sudden, regardless of switch position, I only get air through the vents.

When I move the selector from position to position, I can hear all the little vacuum gizmos changing around in there, but the air still only comes out the vents.

I had a really good write-up saved to favorites, where some guy completely rebuilt his air system with a ton of pictures; but, alas, I changed computers and lost that information; I can't remember if it was here or at FTE.