Dear Car Talk: Your column is the first thing I read every week. Late last year, "Bill" wrote in about his 1999 Altima. He was told he needed a new charcoal canister for his fuel system. Bill complained that he could not effectively top off his fuel tank. He used the words, "I can never fill it up all the way ... even with multiple squeezes of the nozzle." As you know and have preached before, topping off is what kills the evaporative control charcoal canister in the first place.You wryly highlighted the health problems he is foisting on his community, but simply replacing the canister will not necessarily solve the root problem of Bill's blind behavior: topping off. Could you please tell Bill not to top off? Thanks! — Mark


Don't top off, Bill!


Actually, you left out the part of his letter where Bill told us "when I refuel, the gas pump shuts off early."


That tells me that he's not "topping off" when the tank is full. He's simply unable to fill up his gas tank.


I took his letter to mean that if his tank holds 16 gallons, the pump was shutting off after adding only 8 gallons or 10 gallons.


So I don't think Bill is a serial topper-offer. I think he had a genuine problem just filling his tank.


And we agreed with his mechanic, that his vapor recovery system was probably at fault, and a new charcoal canister made sense.


But generally, you are 100% right, Mark. The proper gas station etiquette is to fill the tank until the pump shuts itself off automatically, based on the back pressure coming up the filler neck.


And then remove the nozzle, take your pack of Twizzlers, and leave the gas station.


Excessive topping up is harmful. By excessive, I don't mean when the pump stops at $37.80, and you round up to $38.00. Excessive is when you continue to force more gasoline in, multiple times, after the pump has shut off.


That can send raw gasoline into the car's vapor recovery system, which can cause hundreds of dollars' worth of damage. As a repair shop owner, I really appreciate when people do that. But you might not when you get your repair bill.


So we don't recommend topping off, Mark. Unless you're at the buffet.


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