Why it always makes sense to trust the pump.

Dear Car Talk

Dear Car Talk | Dec 01, 2007

Dear Tom and Ray:

My Ford diesel has a warning about not overfilling the fuel tank. Precisely what would happen if you did this? Would damage to the engine or other systems occur? Most of us tend to "double-click" the automatic-shutoff fuel nozzle after it stops. Is this too much? Does this adversely affect performance? -- Ray

TOM: I don't think it has any effect on your truck's performance, Ray. But it might affect the performance of your pants. If you spill diesel fuel on them, they probably won't last as long. Plus, with that smell on your clothes, you'll never get a date.

RAY: In gasoline-powered cars, you're not supposed to overfill your tank because the excess gasoline can enter the charcoal canister that's part of the emissions system and render it ineffective. But I can't remember ever seeing a diesel with an evaporative emissions system.

TOM: So I'm guessing that they want to prevent you from sending diesel fuel out through the overflow vent of the fuel tank. For obvious reasons, nobody wants to see diesel fuel spilled, nor do you want to pay for fuel that's on the ground under your truck.

RAY: So for all kinds of cars, it's best to trust the fuel pump. When it clicks off, that's it.

TOM: Right. If you top it off every time you refuel, you might save yourself one extra trip to the gas station during the course of your lifetime. That's hardly worth it.

RAY: But you'll be likely to spill some fuel or ruin your emissions system at some point. Or a perfectly good pair of chinos.

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