Q.

-Greetings from Doha, Qatar. Six months ago, I bought a used 2006 Hummer H3 (now with 115,000 miles), which my wife uses five days a week to take our son to school. Traffic in Doha is notoriously bad (and a bit dangerous), so these daily school runs are pretty stressful. But we may have a safety valve. We can trim the commute (and avoid a lot of traffic) by taking a three-mile off-road stretch. This isn't boulder-strewn terrain, it's just mildly rocky dirt. I encourage my wife to take the H3 on this stretch at every opportunity - but she's concerned that 60 off-road miles every week (to and from school twice a day, five days a week) is too much for the car and it will "fall apart" prematurely. We're only going to be in Doha for another 18 months, and she wants the vehicle to last at least that long. I say it's a Hummer - and that this kind of driving is a Hummer's existential purpose. I don't see the car falling apart for several years to come. What do you guys say? Can she take our H3 off-road on every school run?

TOM: I think the real problem is ride quality in the Hummer. She's bouncing off the seat on that dirt road, banging her head against the dome light. If you had spent a little more and bought her a Cadillac Escalade, you wouldn't be hearing these complaints.

RAY: You're actually both right about the H3, though. Sometimes when you see a used-car ad, the seller will say something like "150,000 mostly highway miles." Why does he say that?

TOM: So he can unload his rattletrap of a car.

RAY: Sure. But it's well known that smooth highway miles take less of a toll on the car than bumpy, pothole-encrusted city miles.

TOM: When you bounce the car up and down off the road continually (which is what you're doing on a rocky dirt road), you cause certain parts to wear out more quickly - notably, the front suspension components.

RAY: Things like the tires, the struts, the springs, the ball joints, the tie rods, the stabilizer links, the steering rack and all the bushings and couplings down there get pounded when you drive that way. Those parts will wear out sooner than they would have otherwise. Even on a Hummer.

TOM: All that shaking and bouncing also causes other stuff in the truck to "loosen up." This ultimately results in what we call "rattletrap syndrome," where even if stuff is still working, the truck sounds and feels like it's falling apart because it rattles and clanks down the road, even on pavement.

RAY: In truth, it's impossible for us to know if something major will break in the next 18 months, whether you take that shortcut or not. The car has a lot of miles on it. And who knows if it would have happened anyway? All we know for certain is that the chance of having to replace suspension components goes up when you move from paved roads to rocky dirt roads.

TOM: If it were me, I'd take my chances. The H3 is based on Chevrolet's small pickup truck, the Colorado. So while it's not super heavy-duty (or unbearable to drive) like a real Hummer, it's based on a truck chassis, and should be able to take a reasonable amount of punishment.

RAY: And based on what you say, the shortcut would save your wife time and stress, and might even be safer, since it's less heavily trafficked.

TOM: I agree, as long as your wife has good cellphone service that works out there. If she ever were to break down, or just get a flat tire, you'd want her to be able to easily and reliably call for help. And make sure she has a broomstick or something in the car, so she can fend off an angry band of camels if necessary. Good luck.

Q.

-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.

TOM: First, stop hanging around your parked truck so much. 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. 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.

Got a question about cars? Write to Click and Clack in care of this newspaper, or email them by visiting the Car Talk website at www.cartalk.com.