TOM: First, stop hanging around your parked truck so much, Grover. People are going to think you’re some kind of weirdo.
RAY: This truck, like all vehicles these days, has an evaporative emissions-control system to keep gasoline vapors from escaping into the atmosphere. I think what you’re hearing is the system’s pump.
TOM: To contain the vapors and prevent air pollution, your fuel system is kept under constant pressure, so the vapors are pushed into a charcoal canister, where they’re trapped and held. Then, when the engine starts, they’re released into the cylinders to be combusted.
RAY: That fuel-system pressure is the reason you sometimes hear a little “whoosh” of air escaping when you unscrew your gas cap.
TOM: The system has a self-test mode. It’s possible that all you’re hearing is the pump pressurizing the system to test it. It may have just been a coincidence that you’ve been near the truck every time that happened.
RAY: But I’m guessing that the pump is coming on more often than it should. That’s probably because your system isn’t holding pressure.
TOM: You could have a slow pressure leak somewhere. It could be from your gas cap, a bad valve, a rusted fuel-filler neck or a dozen other places. But something’s allowing the pressure inside the fuel system to drop, and that’s kicking on the pump.
RAY: Or, the pressure sensor is faulty, and is turning on the pump when it doesn’t need to be on.
TOM: Either way, if we’re correct, the Check Engine light will come on soon – if it’s not on already, Grover.
RAY: Have a mechanic scan truck’s computer for trouble codes. That will give him some clues as to where to start looking.
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