Sucking up dirt from the bottom of the tank myth?
 

You often hear people say that you shouldn't drive your car on a near empty tank because it can suck up dirt from the bottom of the tank.

This seems like an urban myth to me as if there was dirt in the tank, wouldn't it get mixed into the fuel when it's sloshing around when driving anyway?

Just curious to see if anyone has a concrete answer on this?

Whether it originates or collects in the tank is moot.
There is absolutely 100% dirt which can develop and collect in the fuel system in general and create problems over time.

It's why there is a fuel filter and it's why a fresh fuel filter can cure running issues.
Take your pick from this selection of dirty fuel filter images.

I can't see how a low level would result in stuff being sucked off the bottom of the tank, as the fuel is always drawn from the bottom of the tank anyway. But if there was any lighter-than-fuel debris floating on top of the fuel then that might then get sucked in as the level got really low?

But yes, there's a filter! Funny how they seem to collect a lot more dirt on a diesel car... I cut open my Accord diesel filter after ~85k miles when I got DTCs for low fuel rail pressure and it appeared to be practically blocked, however we've just sold our 2003 MR2 which we owned from new and in 90k miles the filter was never replaced even once.

In colder weather when parked up, the less fuel there is in the tank the more condensation will form on the walls of the tank that aren't covered by fuel....I would think, reason enough to keep it topped up?
Most pumps these days are "in tank" and will use the fuel as a cooler to a certain extent, it'd be safe to assume that a pump which spends more time immersed in fuel will therefor last longer?

That's my take on things !

I have worked very closely with fuel and even 10ml of water in 10l of fuel is visible, water is substantially heavier and will sink straight to the bottom.

It is all largly nonsense however, I drive with my fuel to the light then top it up. If you are worried simply ensure the filter is changed on the interval.

You make a great point regarding water however, likly more of a problem than dirt, especially if the temperature drops as it will be a small frozen block.

I could be wrong but i think its more along the lines of running it empty, not just to the light on, then fill.

Even with that in mind, my 51, 52, 54 and 08 lagunas i used to wring dry as they would last about 60 miles on warning light, never had a fuel issue, many other issues yes (but thats par for the course with renault :rolleyes: ) but never fuel, when i picked this V70 up, it was ran down to the last 25ml, the jalopy corner garage evidently didnt want to waste any more money, this does have an issue thats sort of fuel related, i have to give it gas to start it, been a long time since ive had to do that with a car...

This is always a problem in the marine world, many boats develop fuel problems in the spring due to water in the fuel tanks overwhelming the filters and separators, the answer is to always fill the tank to the brim when 'winterising' the boat as it eliminates condensation forming on the walls of the tank, modern biodiesel is especially hygroscopic.
Failurn to keep water out of diesel can result in the dreaded diesel bug, A contamination of bacteria which turns fuel into a smelly black jelly substance which blocks filters and clogs lines, some of the bacteria involved are — clostridium; desulfotomaculum; desulfovibrio; flavobacterium; hydrogenomonas; pseudomonas; sarcina. All very nasty and surprisingly common.

my view on this (not with any level of expertise) is that if you always run the tank (close) to empty you'll never have a build up of 'dirt' as all the fuel is filtered & used ... you'll only get accumulation if you never empty the tank.

The issue with water/condensation and growth in fuel is only an issue of you do low milage and your tank sits with low fuel for a decent period, if refilling from empty regularly this can't be a problem.

If you take on a vehicle that has been filled with dirty fuel, always topped up and never run 'dry', when you run it low all the cr*p will come through in one go and quickly clog filter etc, if its going to happen. change the filter - no problems.

(going back a few years) when i got my my first car (mini) it would only take about 3.5 gals from running dry to full, after a few months of me running virtually dry at each fill up i could put the full 5 gals in the tank at fill up - 1.5gals of 'dirt'?

a total myth ... all the old carburettor cars have a filter screen on the pick up pipe and for the last 30 years the fuel is picked up from a secondary 1 litre "pot" which holds a fuel pump with it's own filter mesh . Whether the tank is full or empty if there is dirt in the tank it cant block anything ...
I have seen the insides of many many volvo fuel tanks and i have never seen anything which resembles serious dirt in the bottom , plenty of petrol in diesel tanks though !
No need to worry about it .. It is wise never to run the gauge past the very lowest mark though ... you could run out of fuel at any moment 😺

Had this very thing a few yeas ago on our then '03 S40 diesel where it had attached itself to the gauze pick up...see here.

You could clearly see until those nice folk at BucketPhoto sabotaged our photos. I can add the photos if needs be.