Your fuel pump is leaking, and that's why you're only getting a lousy ten miles to the gallon.

Dear Car Talk

Dear Car Talk | Feb 01, 1992

Dear Tom and Ray:

Our father (small f) recently gave us an old 1978 Datsun 510 station wagon with an automatic transmission. It runs well, but we get less than ten miles per gallon. There is a very small leak of gasoline coming from under the fuel pump, and whenever we stop for gas and remove the gas cap, a forceful rush of air and a fine spray of gasoline (about half a cupful) gushes forth. The garage has ordered a new fuel pump, which is on back order because it's hard to locate. Could there be something else that's causing so much pressure on the fuel system? Maybe there's nothing wrong with the fuel pump, and it's just leaking from such high pressure? If you can solve our problem, we'd invite you out for dinner, but Mother Superior doesn't let us go out after dark!
Sisters Tracy and Michelle

TOM: It sounds like you have two problems, sisters. First, your fuel pump IS leaking, and that's why you're only getting a lousy ten miles to the gallon. The rest of the gasoline is pouring out onto the street. You should replace the pump as soon as you can. And if your mechanic can't find one, have him throw in an electric fuel pump. It'll work fine, as long as it meets the same basic specifications for pressure and volume.

RAY: Your other problem is the fuel tank ventilation system. When you're driving the car and the fuel pump is sucking gasoline out of the gas tank, you have to let fresh air come in to equalize the pressure. It's just like drinking soda pop from a bottle. You have to leave a little space between your lip and the bottle so that air can go in and replace the liquid that's come out.

TOM: Similarly, if the fuel tank ventilation system is restricted somehow, then enough fresh air can't come in and a vacuum is created inside the tank. That's what's wrong with your car.

RAY: So when you take off the gas cap, you hear a big "whoooosh." But it's not pressure from inside the tank pushing air OUT, it's vacuum pressure pulling outside air IN. And that incoming air dis?places some gasoline, which promptly sprays itself all over your habits. So I definitely wouldn't go around smoking any cigars after refueling this heap (we've heard how much you girls love those El Productos).

TOM: Replace the fuel pump as soon as possible, get the fuel tank ventilation system fixed, and you ought to be all set. Good luck, sisters.

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