Drive it forever

Q. My next-door neighbor tells me I should add a quart of automatic-transmission fluid to my 2002 Corvette's gas tank before I put it away for the winter. He says that this will keep the gas from gelling.

Is this another old wives' tale?

A. I think he's confusing a couple of good ideas from olden times.

It used to be common to add a small amount of oil, often marketed as "top oil," to fuel tanks to provide extra lubrication for the valve train. But this became less and less necessary, starting as long ago as the 1960s - back when valve guides were simple holes drilled into an iron cylinder head. Lately the guides, bronze sleeves pressed into the head casting, are more wear-resistant.

When we transitioned to unleaded fuel, there were major concerns about valve-seat recession caused by the lack of lubricating lead compounds in the fuel. Additives to fix this problem are still on the shelves.

And it's a common practice to add antigelling additives to diesel vehicles to keep the fuel in the tank from turning into big, waxy flakes in cold weather.

None of this stuff applies to your relatively new Corvette. Moreover, adding transmission fluid to your tank would likely contaminate the oxygen sensors with zinc or phosphorus compounds. These are extreme-pressure additives, necessary because the transmission has plenty of steel gear teeth sliding over one another, which is not really a concern in the engine.

If the vehicle is going into storage for a few months, however, it should get some fuel stabilizer, which will keep the gasoline from oxidizing and turning to varnish in the lines. I recommend it for any engine, and that includes motorcycles, chain saws, outdoor power equipment, boats, personal watercraft, and anything else that runs on gasoline.

Add the product to the tank, fill immediately, and then run the vehicle for a few minutes to get the stabilized fuel all through the system. Follow the directions on the package as to how much to add.

Q. I have a 2003 GMC Sonoma. The engine roars loudly, but when I depress the clutch it stops. The truck shifts fine, but when the clutch is not depressed and I'm going down a highway, it's very noisy.

Someone told me that it was the throw-out bearing in the clutch. If so, how hard is that to replace?

A. Sounds like more than a throw-out bearing, which would not make noise when your foot is off the clutch. I think it's a "walking crank," resulting from worn-out thrust bearings.

Those worn bearings cause too much crankshaft slop, allowing the crank to move back and forth in the engine block farther than the usual five-thousandths of an inch or so fresh bearings allow. Pushing down on the clutch pedal forces the crank all the way to the front of the engine block, quieting the noise.

Here's how to check: With the engine idling, watch the front crank pulley while someone steps on the clutch and quiets that irritating noise you're hearing. Remember, don't lose anything important like your wedding ring - or the finger that it's attached to - by getting too close to the fan or to any of the belts. If you can see the pulley moving toward the front of the vehicle as your pal steps on and off the clutch, you've got a major engine rebuild coming.

Not the problem? The flywheel could be loose on the crank, or something might be awry with the clutch-pressure plate, which at the very least will require removal of the transmission.

Whatever it is, it's not good. Don't drive this truck, not even to the shop. Sorry.

Mike Allen is a senior editor for Popular Mechanics magazine. Questions should be sent via e-mail to driveit@nytimes.com.