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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?

Pᴀᴜʟsᴛᴇʀ2
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HandyHowie
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1 Answers1

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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.

TMN
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vini_i
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    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 '16 at 23:11
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    @HandyHowie No fuel pumps were harmed during the making of this post. – vini_i Feb 19 '16 at 02:54
  • 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 '16 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 '16 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 '16 at 12:44
  • @IhavenoideawhatI'mdoing I'm not saying that the fuel heats up the pump, i'm saying that the temperature of the fuel is high enough that the fuel won't draw away the heat that is already there. The fuel does get compressed although just slightly because it leaves the pump at a higher pressure then when it came in. The action of passing the fuel through an impeller or gear rotor is what heats the fuel. It is knows an an adiabatic process or without adding heat. This is similar to hitting a penny with a hammer. The penny gets hot but no actual heat was added. – vini_i Feb 19 '16 at 13:00
  • @IhavenoideawhatI'mdoing Let me rephrase it a different way. The amount of heat that the fuel passing through the pump draws away is very small when compared to how much heat the fuel that the pump is submerged in draws away. – vini_i Feb 19 '16 at 13:05
  • @vini_i I understand what you're saying, but it doesn't seem right. Fuel will draw away heat that is generated by motor friction. Fuel practically shouldn't change in volume, even if it exits at higher pressure. You would need tremendous force to compress liquid. You would be correct if we were talking about gasses, though. A penny gets hot mostly because of friction between metals, unless you reduce it's volume or change it's shape significantly, I understand the principle of an adiabatic process. – I have no idea what I'm doing Feb 19 '16 at 13:11
  • I understand that the fuel in which a pump is submerged in takes away much more heat than passing fuel, but it is enough. Some designs have the pump high enough that the pump is only submerged when there is a lot of fuel in the tank. Which in your case would mean every time you pass the half tank mark your pump would start boiling. Automotive engineers wouldn't make such a blatant mistake. – I have no idea what I'm doing Feb 19 '16 at 13:13
  • Let us continue this discussion in chat. – vini_i Feb 19 '16 at 13:16