Hey

if any one can help it would be very much appreciated. the problem is i have a F-150 an 03 model my ac is cooling very well but my issue is when i excelerate the air moves from the ac vents to the defrost vent. you let off the gas peddal and it comes back to the ac vents along with it goes from blowing cold air to warm air during this time. i had a mechanic who so called fixed it and said it was a vacumn leak to which it did help it but not totally fix it. Now it does the same issues but mostly at speeds between 40 and 55 it doesn't seam to do it as bad if any higher or lower. i have since brought it to another mechanic he said i still most have a vacumn leak so he out a smoke thru my engine while i was there and neither of us saw any smoke leaking from any where. So i guess my question is could there be some thing internal in the engine or transmission that could be making me loose vacumn or is it totally some thing different than vacumn now. thanks for any help any one can offer ..........and i also have replaced the selector switch in the dash hoping it might help.

 

This is a vacuum problem still. These trucks have a plastic cannister that sits behind the battery, mounted to the back of the tray that holds vacuum just for this and the 4x4. On a nice, long acceleration up a hill, it will eventually loose its vacuum, but not for a good while. The biggest culprit is a vacuum checkvalve that is leaking, which, if you open the hood and look at the bracket that holds all the wiring harnesses to the left rear of the eninge compartment, just to the left of that mess, but before you turn the corner into the battery tray, it's usually contained right about there. Sometimes, these truck will develop a kink or leak in the vacuum supply line that goes inside to the control head. This area i mentioned is also a very fertile area of wiring problems, since the battery will sometimes (if not maintaned well or there is a charging system concern) spray the area will battery acid. Also, it really helps if this is the first time to be taken apart in a few years, that you go ahead and take a look at all the lines. They get very brittle and will usually break anyway, which requires repair.