Why topping off the tank is a bad idea.

Dear Car Talk

Dear Car Talk | Dec 01, 2008

Dear Tom and Ray:

I got into an argument recently concerning whether I should top off when I fuel up. You know what I mean -- the pump shuts off, and I can either stop or put another half a gallon more in. Is that OK, or not? I notice some pumps say not to top off, and I was wondering why. Most reasons I have found online discuss fuel expansion, which, it seems to me, would be rather minimal. So now I am asking the experts ... can you point me in the right direction? -- Kurt

RAY: Was the "argument" with a spouse, by any chance, Kurt? Who else would you ever get into an argument with at a gas station? Anyway, you owe her an apology, Kurt.

TOM: Vaporization is the problem. In the old days, gas caps used to have holes in them so gasoline vapors could escape. The vapors went right into the atmosphere, creating smog and preventing Sarah Palin from seeing Russia from her home in Alaska. Oh, and the other problem with smog: lung disease.

RAY: To solve the problem, the Environmental Protection Agency required that all cars come with a vapor recovery system. A key piece of that system is something called a charcoal canister, which is attached to the gas tank.

TOM: When the gasoline in the tank expands and creates vapors, those vapors are absorbed by the charcoal. They're held there in the charcoal until the next time the car starts, and then the vapors are sucked back into the intake air, and burned by the engine during the combustion process.

RAY: When you top off your tank, you take the chance that you'll force liquid gasoline into the charcoal canister, ruining it. And what does that mean? At least 300 bucks, Kurt.

Get the Car Talk Newsletter

Got a question about your car?

Ask Someone Who Owns One