Pump Down the Mileage

Feb 02, 2019

RAY: A fellow comes into the shop and he complains that his Volvo, an older Volvo, is misbehaving. It seems to lose power. It sputters. It has difficulty climbing hills. It's especially bad if the tank is less than half full. We say, huh, piece of cake. It's a classic problem of the feeder fuel pump being kaput. These Volvos have a pump in the tank, which is a feeder pump, which pushes the fuel to the main fuel pump, which is located outside the tank underneath the car.

So when the feeder pump fails, the car will still continue to run because the main pump will actually pull fuel out of the tank rather than have it pushed and assisted by this feeder pump. But it won't work great, and the car will suffer from poor performance because it hasn't got sufficient fuel pressure.

Classic symptoms. So he gives us the go ahead to replace the pump. Off he goes. A week or two later he returns and says, “Geez, the car runs great, but my mileage is down. You must have done something,” and we say, “Of course not. How can we have done anything to affect your mileage?” But he says it's off at least ten percent.

A few weeks later he comes back and says it's down even more, maybe 15 percent and getting worse. As a courtesy, we check his emissions, which are perfect, and check the timing and all that stuff. And then we throw him out. And we come to realize when he returns for the third time with his lawyer that he's right.  Even though we did nothing wrong, putting in the new pump made his mileage drop.

The repair was done correctly. The feeder pump was installed correctly. And yet by putting in the pump, we reduced his gas mileage. How could this be?


RAY: What we missed is that there is a piece of tubing that goes between the tank pump and the main pump, and it had a little pinhole in it. With the intake pump broken, this little pinhole did not cause gasoline to leak out because there was suction created in that tube. When we replaced the feeder pump, we created positive pressure in that line between the two pumps. Even though most of it was being sent to the injectors, just enough was leaking out through this little pinhole to cause this drop in mileage. So he was right, even though we didn't do anything wrong.

Get the Car Talk Newsletter