Dear Tom and Ray: My name is Luke, and I am a musician playing double bass with a band based out of Norman, Okla. Our record label has thoughtfully provided us with a Ford E350 van for touring. Recently, we have upgraded our trailer to a larger and heavier one. While this makes it much easier to transport our gear, we've noticed an issue occurring with our air conditioning. With the trailer attached, the AC will cut out whenever we accelerate or travel up an incline. What's worse, when the AC cuts out, the heat automatically kicks on, full blast. We've taken the van to mechanics a couple of times and they say that they've never heard of this problem before. It appears to occur more frequently as the temperature rises and we need the AC more. This van has AC controls for both the front and back sections of the van, and this is occurring with both sections, usually at the same time. Since we are on the Texas country circuit, we naturally spend a lot of time in Texas. Having nine men in a van with no AC (or worse, the heat on full blast) in southern Texas leads to astounding levels of … interesting odors. I am composing this letter in the van, sweating profusely.
Ray: In addition to a perspiration problem, you have a vacuum problem, Luke.
Tom: There are little "blend doors" in the ducts of the ventilation system that open and close to direct hot and cold air to where you want it. Those doors are held open or closed by small vacuum motors.
Ray: And for some reason, those motors aren't getting enough vacuum, especially when the engine is under a condition we call "WOT," or "wide-open throttle."
Tom: The vacuum is created by the engine, when the pistons go down and suck air into the cylinders. Vacuum is at its highest when the engine is idling and the throttle is closed (closing the throttle is like putting your hand over the end of your vacuum cleaner's hose) and at its lowest when the throttle is wide open.
Ray: So, either you have lousy vacuum to begin with and it's failing under high demand, or you have adequate vacuum and it's leaking out somewhere along the way.
Tom: So, start with a simple pressure test to see if your engine is producing enough vacuum right at the intake manifold. That's the source of your vacuum, and it's from there that vacuum gets distributed to the heater controls, the power-brake booster and anything else that requires it.
Ray: If the vacuum is sufficient there, then you'll need to trace the route of the vacuum from the engine to the blend doors. There could be a crack or a small hole in the manifold itself, or in one of the vacuum hoses.
Tom: Or you could have a bad vacuum reservoir (which stores vacuum for use during those moments of wide-open throttle so you don't get blasted with heat).
Ray: Some systems even use a check valve to maintain vacuum during wide-open throttle, and yours could be broken.