My '99 F150 (5.4, auto, 4x4) began acting a little strange the last few weeks in reference to the title of this topic. Today I found what I thought to be the problem. There is a series of small vacuum lines to the rear of the battery. The one leading through the firewall was broken. It looked like it rubbed on the neg. battery cable clamp. I spliced it with a piece of new hose and some duct tape, (I know, I know :o ), for now.

Problem is that under medium to heavy acceleration the air switches by itself to the defroster, regardless of what I have selected. (Another example is when I'm climbing a semi-steep grade.) Before I found the broken vacuum line, I could only get air through the defrost vents. I don't know if my repair isn't sealed properly or if there is a bigger problem lurking under the dash. I've searched through many posts over the past few days and cannot find an answer to this problem for my year vehicle.

I do plan on replacing the broken line, just need a part number or a name to give to Ford.

Any help would be appreciated.

Thanks in advance!

 

It's a vacuum leak.

I just went through this with my '97 Expy. One of the nipples was busted off the manifold located between the firewall and the battery. Fortunately I had enough of the nipple left where I could make up and ziptie the hose back on.

 

My problem is that the vacuum line actually broke in half, more liked sawed in half, about 3-4 inches below the connector. I think the only way to fix it is to replace the entire line. Just don't know how to get to it inside the cab.

 

most auto parts stores will have vacuum line repair kits. these work quite well in most cases. good luck with the fix

 

I picked up some 7/32 (I think) vacuum line and spliced it back together. I did get the Ford part # also, if anyone is interested. The repair seems to be working so far. We'll see in a few hours while I drive through the mountains from San Diego to Tucson.