What happens when gerbils power your car?

Dear Car Talk

Dear Car Talk | Dec 01, 1988

Dear Tom and Ray,

I have a 1977 Toyota wagon that has turned out to be a great car in all respects except one. Once a week or so, it behaves just as though it were running out of gas. It bucks and falters, then runs smoothly, then bucks and falters--but never quite dies. It has plenty of gas. However, if I pull over and take the gas cap off--even for just a moment-- the car runs like a champ! Do you have an explanation for this?


TOM: These 77 Toyota wagons are actually powered by four gerbils running on a big exercise wheel under the hood. Every once in a while, they run out of air, and when you take off the cap....

RAY: Well, believe it or not, my brother is close. It's not the gerbils that need air, it's the gas tank. Fuel systems must ventilate. In the old days, when my brother was middle-aged, a pin-hole in the gas cap allowed vaporized fuel to escape and fresh air to enter. This was mechanically sound, but environmentally screwy. Your Toyota has a much more sophisticated ventilation system. Your vaporized fuel is adsorbed in a charcoal canister. When the car is started, the vaporized fuel is sucked back into the engine along with fresh air.

TOM: If the ventilation system is clogged, however, and no air is getting in, the pressure in the tank decreases. Did you pay attention in Physics class? If the tank cannot equalize the pressure by sucking air through of the ventilation system, it starts looking for other places to draw from. In your case, what it's doing is sucking back the fuel that is heading for the engine. When the pressure gets low enough, you get the bucking and faltering. As soon as you take off the gas cap--and hear that whooosh--the pressure in the tank goes back to normal and the car is fine again--for a while.

RAY: Have your mechanic check the gas tank ventilation system and repair whatever component is malfunctioning. But just to be sure, check under the hood for any Habitrail boxes or little brown pellets.

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