1998 Ford E350 A/C Issue

[vermilionfox]

I have a 1998 Ford E350 with the 5.4L and front and rear air. I'm having an issue with the A/C. When I'm on the highway towing my boat (22 foot pontoon) and the engine comes under a load the air flow drops from the dash vents down to the floor. This does not happen when I'm not towing. Vacuum leak? Where do I start looking?

[Scott K]

It is vacuum controlled, and more then likely either the vacuum tank has a small crack in it, or the vacuum check valve is bad. You could hook a vacuum gauge to the black line that goes into the vehicle under the passenger side foot area, see if it drops. Then measure the line before it goes into the vacuum reservoir under the hood. If it drops inside, but not out side it is a bad reservoir or vacuum line , if both drop, it could be the vacuum check valve.

The vacuum reservoir is inside the evaporator case, and it is easier to just add an external one somewhere, rather then replacing the existing one.