When the low fuel light comes on, how much fuel is typically left in the tank?

[Tiffer E38]

Originally Posted by Nlambert

It's pretty silly to me to run until the gas light comes on. Fuel pumps require submersion in fuel to help cool them off.

Why this keeps getting repeated is beyond me. As has been pointed out a few times in this thread, NO, they don't. They are cooled by the fuel running through them (many are not even in the tank and the ones that are, are usually up near the top with the pickups down at the base, so they spend 3/4 of the time not IN the fuel anyhow). So long as you don't actually run out, you wont 'have a problem with the fuel pumps "burning out." And no, there's only sludge at the bottom in older cars and even then, the fuel pickup is usually at the base of the tank so if there's sludge in the tank, it'll be picked up even when the tank is full!

[Arcenal813]

Originally Posted by bigpaul

it is utter stupidity to run a vehicle until the fuel warning light comes on, when you pour in the new fuel it will stir up all the crud in the bottom of the tank, I did this over 30 years ago and it was a heck of a job to restart the vehicle, I've never done it since, I now refuel when the tank indicator gets to half.

Not really "stupid." When adding fuel system cleaning agents to the tank, it is actually suggested that you run it until about the 1/8th mark for several reasons, INCLUDING burning off said crud to clean out the tank. In other words, once in a while, it's actually wise to let it get low (not necessarily to the Fuel light on point, but close).

[Nlambert]

Originally Posted by Tiffer E38

Why this keeps getting repeated is beyond me. As has been pointed out a few times in this thread, NO, they don't. They are cooled by the fuel running through them (many are not even in the tank and the ones that are, are usually up near the top with the pickups down at the base, so they spend 3/4 of the time not IN the fuel anyhow). So long as you don't actually run out, you wont 'have a problem with the fuel pumps "burning out." And no, there's only sludge at the bottom in older cars and even then, the fuel pickup is usually at the base of the tank so if there's sludge in the tank, it'll be picked up even when the tank is full!

*sigh*




An internal fuel pump gets mostly cooled by being submersed in liquid. It gets lubricated (which does help cool it to a point but not enough) by fuel flowing through the pump. In-tank pumps are in a sealed environment where there is no fresh air to circulate to help keep the pump cool. As the air in the tank heats, so does the pump. In short they are liquid cooled.


An external fuel pump gets lubricated by fuel running through the pump. These pumps get their cooling from being in an open air environment where two things happen. 1) Circulating air helps cool the pump and 2) the pump can radiate heat away from it. These pumps are air cooled.


On diesel engines they use external fuel coolers (radiators) to help keep the temperature down.

As for the fuel pickup you are correct in that it sits in the bottom of the tank. That is why there is a sock on the bottom of the pickup. It filters most contaminates and whatever is missed by the sock (or screen) is caught by the external fuel filter before it reaches the engine.


Only one other person in this thread seems to think that the fuel surrounding the pump doesn't cool it. The other person who said their pumps aren't cooled by being submersed is because they said their vehicles run external fuel pumps, which again, are air-cooled. Different animal altogether.



Go talk to any mechanic worth their salt and you'll get very similar answers.