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Fuel pumps and fuel economy

 
author and iconoclast
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My car has one of them nifty little com-pYOO-ter things in it. Tells you the average gas mileage, among other things. When you reset it, it resets the calculation, and the numbers waver a lot at first, then settle down. I end up pushing that reset button a lot, because trying to drive so that the number goes up helps me avoid jackrabbit starts and other nonsense.

So over the last month I noticed a 3-4 MPG drop in fuel economy. Not so drastic that I immediately took it to the garage, but a real and noticeable change. I was just starting to become convinced that something was wrong with the old girl when she starts running real rough, splutters, coughs, and dies. She'll turn over, but won't start up again -- just more spluttering and coughing.

To make a short story even shorter, it was the fuel pump. As soon as they replaced it and I drove out of the service station, I noticed two things: more pep power, and improved mileage, back to the way she used to be.

So my question is, if you're still with me: ol' paint wasn't leaking gas, as far as I could tell. So how could a bad fuel pump cause low power and low fuel economy, both? If not much fuel was getting through, you'd see low power but if anything, the mileage should go up, it seems to me. You?
 
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If my understanding of these things is correct -- and that is quite suspect -- when fuel isn't delivered at the proper rate, it burns less efficiently. So I think a lot of your gas may have just been spewing out your tailpipe.
 
Ranch Hand
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Correct. The mixture determines the fuel economy. If not enough fuel is delivered per stroke the power goes down as well as part of the fuel that is delivered not combusting.
Revs also go up to compensate for the lower power (you press the accellerator harder to get your performance anyway), so you pump more fuel through that pump as well.
 
(instanceof Sidekick)
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My gas mileage was going on the ground. The filler tube from the gas cap to the tank rotted out. There was so much mud & debris jammed up in the fender that it didn't puddle on the ground, but probably soaked up a perfect little explosive package that I carried around until it started to smell. I'm up about 5 after fixin it.
 
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