[Dr. Dyno]

I recall covering the fuel pump cooling issue elsewhere recently so here I'll just dispel the myth that it must be submerged to stay cool. It's the fuel running thru it that does the cooling. Many cars like our RAV4s have wide flat tanks with the actual pump taking up about 1/2 the height. The pump starts being uncovered below maybe half a tank. If cooling was that vital they'd have to be in a sump in the bottom of the tank.

Some cars have the fuel pump in a canister that keeps the fuel pump submerged in fuel until the last quart or two of fuel remains in the canister. The tank can be empty before the the pump drains the canister.

If this was true what keeps the level in the canister higher than in the tank? Magic?

For years I've run our cars 20-50 miles AFTER the low fuel light comes on. My 17 gallon Accord tank regularly takes 16+ gallons.

 

[Dr. Dyno]

There are several ways to keep the fuel pump canister full, a small auxiliary pump that drew off the bottom of the tank to keep the canister filled,

Yeah, that would make a lot of sense. Add another pump you have to keep cool.

these were used by Saab over the years

And what happened to Saab?

One way or other it does not take much pressure to move fuel a few inches from the bottom of the tank up into the canister.

Hmmm, maybe someone could use several of these schemes to eliminate the pump altogether and get fuel all the way to the engine, a few inches at a time.

return fuel from the injection system dumped into the canister,

That could be easily done and I believe it is. W/o having one in hand I can't be sure where the return fuel goes but the pictures of pump assemblies on eBay show a plastic housing the pump must be inside of. In any case the fact remains that keeping the tank partially full to prevent the pump from overheating is a myth.

 

[Dr. Dyno]

I'm sure if you ask a fuel pump engineer about what is better for a fuel pump, low or high fuel they will always side with high. Fuel cools (like you stated) and lubricates. ...

Point is it is better to have a full tank for the fuel pump, but nit picking the car now weighs more, the car has to work a little harder and use a little more fuel to move it (though it is miniscule)

Now about Saab, what have you got against them?
Tyler

Definitely agreed on all points. And thanks for writing it up.
As one of those design engineers (electronics) I only want to counteract what are sometimes entrenched myths. To me it makes no sense to carry around half a tank of fuel you'll effectively NEVER use. But it is true you'd be hard pressed to measure the difference in fuel mileage or acceleration you lose by doing it. Yet the benefits of weigh reduction are the reasons some cars have replaced the spare tire with a can of emergency tire "fix." But then how many owners cancel all that out by using their cars for storage?

Our RAV4's low fuel light is probably on now and I'll be bringing it discounted gas in 5 gallon cans when I make my trip to fill up my Accord. It's just too bad I have to make a fuel run with 1/8th tank still in my Accord.

I don't know where you got the 20 psi, :shrug: but I do have to question the extra electrical power needed to pump fuel up from a low tank. While it may be real in concept I doubt you could measure it. In any case I'd bet DRLs take quite a bit more power but no one's talking about turning them off to save fuel.

You got me on Saab! I actually owned three 99s back in the 70s. Great cars and definitely ahead of their time. :thumbs_up:
All mine had analog electronic fuel injection and it worked perfect. In and era of carburetors and temperamental "automatic" chokes the Saabs started and ran flawlessly regardless of the temperature. And front wheel drive when everything but 4X4 guzzler pickups was rear wheel drive ... Then they went to mechanical fuel injection, a major step backwards IMO, and Hondas came out ... I think the rest of the world caught up and I never owned another one.