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Motormouth: Why you shouldn't top off your tank

Bob Weber, Chicago Tribune on

Published in Automotive News

Q: My 2005 Dodge V-6 Caravan (106,000 miles) suddenly refused gas with the pump constantly kicking off, as if full. Checking the internet, I found the problem was a carbon pollution canister above the gas tank. Replacement cost was around $800-$1,000. One mechanic told me to keep the tank at a quarter-full for a month. I did that, and the problem resolved itself about 2,000 miles later.

I'm still pumping with no problem. Can this problem actually resolve itself?

-- R.P., Bath, Pa.

A: The fuel emissions canister is filled with charcoal, which traps and stores fuel vapors to keep them from escaping into the air while the car is parked. When you start the engine, a valve opens to allow those vapors to get sucked into the engine emptying the canister, readying if for the next time.

If you keep pumping gas once the nozzle clicks off, you may be forcing liquid into the canister. After a while, as it vaporizes, the charcoal dries out, but not always. A warning to all drivers: Do not top off your tank.

Q: What happens when I take my foot off the accelerator and just coast? Is any gas being used? Because my cars slow down so gradually there must be very little mechanical friction. Do the pistons ride up and down on a film of oil or is there actual metal to metal contact? Transmission information would be interesting too.

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-- R.C., South Elgin, Ill.

A: Fuel usage depends on several factors. When you lift briefly, the fuel injectors continue to pulse. But at a predetermined engine speed (usually around 1,000-1,500 rpm) the fuel injection shuts off until the engine speed drops or you step back on the gas. If your car has an instantaneous fuel economy display, you may see it climb up to 99 mpg during a long coast.

During this time, the engine is still turning, and hence so is the oil pump. No metal to metal contact. Ditto for the transmission.

Q: The windshield in our new car displays both the speed we're doing and also the speed limit. What transmits the speed limit information? Are there sensors built into the road or what?

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