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Why Does Grover's Truck Run at Random?

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Dear Tom and Ray:

We have a 1999 GMC Sierra that has something that "runs" on occasion when it is sitting in the garage. We have not started the truck in nearly two months, and my wife heard it run just yesterday. What is causing this, and how do I stop it? Does this truck have a random battery discharger, or is it just lonely because we have not run it lately? We have had it since new, and it is in nearly new condition due to the fact that the price of gas is so high. Most of its life has been spent in the garage -- except when some family member moves, or during deer season. It sounds like something is running -- like a pump, maybe? Can you tell me what's running and if there's anything we should do about it? Thank you.

-- Grover

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. It's a good system.

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. That'll be your cue to do, what? Check the engine!

RAY: Yeah. Have a mechanic scan the truck and see what trouble codes the computer has stored. That will give him some clues as to where to start looking. Good luck.
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