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I have frequently heard people blaming engine problems on letting the fuel in the fuel tank get too low, in so doing allowing sediment to be sucked into the fuel pump, which then causes a blockage.

I personally find this very hard to believe for a number of reasons, for example -

1) The fuel will be constantly being "swooshing" around as you drive around corners, so stirring up any sediment, which will then go through the pump and filter.

2) On many cars, the fuel pick-up point is very low in the tank and, if the pump is not inside the tank, the tank outlet may well be on the bottom of the tank, so any sediment would be sucked in no matter how low the fuel is.

3) When you fill up, the fresh fuel will disturb any sediment and distribute it in the fuel, ready to be sucked into the pump.

4) I have owned a number of classic cars and have never seen any sediment in their fuel tanks when removing them from the car.

I will be interested to hear if any one has really seen this and has experience of sediment causing blockages, especially when it only occurred after letting the fuel tank get low on fuel.

My question is - Is it really not advisable to let your fuel tank get low on fuel?

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The problem with running the tank low in older cars is that the fuel cooled the fuel pump.

enter image description here

The fuel pump would just hang out in the bottom of the tank with all the fuel and the fuel would wick away heat. When the tank gets low the pump is no longer covered by the fuel. With little fuel covering the pump, the pump gets hot. The pump wears more when its hot and just doesn't like to be hot. There are instances where filling an empty tank with with cold fuel causes the hot pump to contract and the car not to start.

enter image description here

Most if not all new cars use a fuel pump module where the pump sits in a big plastic cup. That cup is always full of fuel to cool the pump. The cup only runs dry as you completely run out of fuel, but then your only running another minute or two. These vehicles are not affected by low fuel unless you run out completely.

Sediment is not a big problem unless your getting fuel from unrespectable places. As you can see from both pictures, both pumps have a input filter (sometimes called a sock) and both socks sit on the bottom of the tank. If any sediment makes it into the tank it will be picked up if the tank is full or empty.

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  • 1
    I hope you haven't just dismantled your fuel pump just to answer my question? :) A good reason not to run low though.
    – HandyHowie
    Feb 18, 2016 at 23:11
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    @HandyHowie No fuel pumps were harmed during the making of this post.
    – vini_i
    Feb 19, 2016 at 2:54
  • Actually, the pump is cooled by fuel by having fuel pass through it rather than being submerged in it. I believe it acts as a lubricant as well. So if you starve the pump of fuel the internal friction and heat will kill it pretty quick. But that happens only when you are completely dry, not low.
    – I have no idea what I'm doing
    Feb 19, 2016 at 12:30
  • @IhavenoideawhatI'mdoing The pump can't be cooled by the fuel it pumps. In the pump the fuel first passes through mechanism that moves the fuel. That could be an impeller or gear rotor. After that it passes around rotor assembly. The action of passing through the impeller or gear rotor heats up the fuel. Then the hot fuel then can't draw away any heat. The idea is similar to a turbo charger that needs an inter cooler. The impeller whips the air, hearing it and then the compressed air requires cooling.
    – vini_i
    Feb 19, 2016 at 12:40
  • @vini_i I believe friction of moving parts heats up the pump, the fuel itself won't be measurably hotter after passing the impeller (except the heat it absorbs from the whole pump while passing). You can't compare a fuel pump to a turbocharger, as a turbocharger is designed to compress air, increasing it's temperature significantly. Fuel won't be compressed inside a fuel pump and it wouldn't be even if you ran it through a turbocharger.
    – I have no idea what I'm doing
    Feb 19, 2016 at 12:44

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