when I start up a fair sized hill, the air switches from coming out dash vents to coming out defrost vents. after I top the hill it returns to the dash vents. I know the doors are vac controled, but cant figure why a load condition would make the doors move. any help out there?

 

You have a vacuum leak.

Check the lines by the battery, there are some sections close to the battery that rot. Also check the tank behind the battery for cracks, and all the hard plastic lines. If it keeps up you may end up with lean codes poping up from the check engine light.

Defrost is the default position when there is no vac to hold the blend door,so you are loosing vacuum.

 

My '95 Bronco used to do that. My vacuum leak was in a white plactic line (probably 1/8") between the vacuum "can" and the firewall. Fixed the leak and fixed the "flip flop".

 

that might explain a problem from the past two years. In the winter when the temp drops low, it acts like a vacume leak. I had never been able to find it but it went away with the warmer weather in spring.

 

pulled the reserve tank and checked it and hoses all the way back to manifold with a vacuume pump. No leaks showed up. I had no way to check the final rubber plug where the plastic line hooks to manifold. Most all of the rubber hose has been replaced.

Any body got ideas on the next best place to look?

 

Did you test the line all the way to the actuator for the blend door? Or just the stuff under the hood?

Thr fitting can be tested with propane and the engine running, if you hear a difference in the idle when you release (un-lit) prorane then you have a leak.

If your mufflers or cats are starting to clog you can loose vacuum on hard pulls b/c the exhaust system will build up a back pressure. That can be tested with a vac gauge.

Here is a link:http://www.aa1car.com/library/converter.htm

 

Tested from tank to engine, held vacuum from host that comes out of intake. Did not test under dash.

Muffler is a flowmaster and cat is in floor of shop, resting

how do you use propane to leak test?

 

For the propane test you get a small propane torch like you use for soldiering copper pipes. Start the engine and let it idle. Turn on the torch WITH OUT lighting it and basically flood the suspected area of a leak with propane gas. If the idle speed increases then you have a leak in that area. The unburned propane being sucked into the intake through the leak makes the motor run just a little bit faster.

Also you can put a vacuum gauge on the intake and raise the RPM's to about 2000 and see if it holds steady. That may let you know if you have an engine problem, you probably don't though. If the vacuum isn't holding I would do a re-check for a leak.

Since you havent checked under the dash that could be where the leak is, Check where it goes through the fire wall real good.

Good luck
Jim