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Ask!: To top it all off can be hazardous

Published May 28, 2007 at midnight

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Hank wanted to know why we're advised against topping off our gas tanks, especially in the summer. That advice, of course, presumes that we know what topping off means.

Generally speaking, topping off refers to filling a container to the top; in the case of gas tanks, Perry wrote, it means adding fuel after the nozzle has automatically shut off.

Perry and Elmo, another writer, said a danger of topping off is that summer sun and heat can cause gas to expand. If a tank has been filled to the top, expanding gas can leak from the tank, causing a fire hazard and air pollution from evaporating gas.

Fred and Perry wrote that topping off the tank could cause problems with the evaporative emissions system, which Fred says has been on cars since 1980.

The Environmental Protection Agency () says that in areas with ozone issues like ours, gas-station pumps are equipped with vapor- recovery systems that return gas fumes to the storage tanks so they don't escape into the air. "Topped-off" gas could be drawn into this system and returned to storage, meaning you'd pay for gas you didn't get.

And the additional gas could disable the station's vapor-recovery system, contributing further to pollution and maybe knocking out the pump, according to the EPA.

Here's another question:

My friend insists that the Starbucks inside grocery stores tastes different from products made at stand-alone Starbuck stores because grocery-store employees aren't trained by Starbucks. Is there a difference from store to store? - Kelly

I know that grocery-store sites don't always have as many products as the stand-alone Starbucks do, but I don't know about the training. Do you?

Post your answers at the Ask! blog, While you're there, check out the other questions or post one of your own by clicking on the link to the left on the Ask! home page.