Problem might be linked to vacuum
Dear Tom and Ray: I am in a band based out of Norman, Okla. Our record label has provided us with a Ford E350 van for touring. Recently, we have upgraded to a larger, heavier trailer. 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. Mechanics have been unable to reproduce the issue and 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. I’ve considered that we are stressing the engine too much with the trailer and that the van is trying to siphon heat off the engine, but the temp gauge never budges. 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. Please help!
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. And when would the throttle be wide open? When you’re trying to climb a hill in a van with nine large, sweaty dudes while towing enough equipment to outfit the Lawrence Welk orchestra.
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 to those blend door motors.
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 Price: $84,700 As-tested price: $89,175 Mileage: 18 mpg city, 28 mpg highway
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