Something I've never quite understood...

[sam_d]

Okay, I'm not being a smart arse here but this is something I never understood...

People will tell you not to let your fuel run down too low as doing so causes the pump to suck up all the crap and sediment from the bottom of the tank. However, since the pump already sucks up fuel from the bottom of the tank why does the fuel level matter?

Am I missing something?

[pop058]

some of the crud floats and when the fuel level drops, the bad floaty stuff is now at filter level.

[Eevo]

when full the crap is disperse around the tank
when near empty the crap is concentrated and there is a greater chance of it being picked up and sucked through

[JDNSW]

Probably more accurately - when the tank is fairly full, the surface of the fuel sloshes back and forth, but the bottom, where the sediment is lying, only has slight movement of the fuel with braking, cornering etc. But as the fuel gets low, the movement of fuel right on the bottom increases, particularly as part of the actual bottom becomes exposed from time to time. This stirs up the sediment, and some of it gets sucked up by the fuel suction, which is always at least a centimetre or two above the bottom.

[rangieman]

Snake oil [bigwhistle]

[Fifth Columnist]

Snake oil [bigwhistle]
Yup! [bighmmm]
After nearly fifty years of driving I've never experienced problems nor do I know anyone who has.
Old wives tale
(In my 'Series' days I always cut off that gauze filter from the fuel tank pick-up pipe. That caused trouble regardless of fuel levels.)

[Roverlord off road spares]

Snake oil [bigwhistle]

Ya know, If it's plastic snake, you get synthetic snake oil [happycry]

[workingonit]

Been into a number diesel poly and steel tanks and only ever seen a couple of resin grains (something to do with returning hot fuel?).

Once had water, but that's because the previous owner lost the cap and stuffed it with cotton stubby shorts, in the wet season. Went through the whole system.

Mate poured some anti-algae additive into his Toyota diesel tank - reckons took about 6 filters before fuel ran clean. I reckon it was the the additive.

Lift the rear carpet and remove cover plate. Take lid off tank and make a visual inspection (can be a bit difficult to budge).

My guess would concern over sucking air if real low. Not sure how air in the fuel line affects the TD5 - self purging?

If some dirt does get in it is likely to be picked up and caught in a pre-filter along the fuel line, as is the case with the D1.

Petrol tanks, and socks around petrol pumps may get resin encrusted.

If the tank is steel, and If water lies under the fuel against the steel you may get rust. Maybe that is where this idea comes from in the olden days when most diesel tanks were steel.

[Tins]

I don't know about snake oil, but I do know keeping a tank full gives less chance for moisture to form in it. These days that may not matter so much, but boy it used to. Cleaned many a gauze in a fuel pickup that was blocked with something that resembled algae. A full tank allows less scope for this. I guess climate may have a bearing on this. B6 Volvo busses were notorious for it, here in the damp Dandenongs. First symptom was a lack of power. Well, a greater than usual lack of power.

The more fuel there is, the less % of crap per litre. A question for the skeptics: why do you bother with fuel filters?

[350RRC]

There's a couple of things running in this thread:

General cr@p in the tank and water in the tank.

I'll only comment on the second.

A lot of petrol cars with carbs will cope with a bit of water, but diesels won't.

The water comes from moisture that's in the air that fills the tank as fuel is drawn out. A cold overnight and the moisture condenses and sits under the fuel. Water can accumulate like this and eventually be first off the list to be drawn off the bottom of the tank.

If the tank is kept as full as practicable this effect is lessened.

Commercial fishers try to keep their tanks as full as practicable for this reason, especially when the fishing season is over or their quota is caught and their boats are just going to be tied up for a period of time.

If they are full there is virtually no chance any water will accumulate.

cheers, DL

[workingonit]

Not a matter of being skeptical as I accept dirt and other stuff may get in through the filler neck. But then shunted along to prefilters or filters. Tanks are generally drained and refilled on a regular basis keeping up a regular cleaning cycle. Algae generally require sunlight to grow, so if present I would think it must come from outside the tank and external contaminate into the fuel - my guess is it would die in petrol or diesel in the dark and not turn into some algal mat inside your tank. I'm not saying that there will not be a case of serious contamination, but it would have to be real case of neglect or stupidity ie the stubbies in the filler neck - someone maliciously putting sand or something else in your tank. Water is the most likely from condensation (should be easily filtered) or flooding in a river crossing (unlikely if your not already leaking fuel, maybe the breather tube, and volume could overwhelm filters).

If you believe you have a problem, then there are plenty of additive remedies out there. Myself, I'll inspect my filters, look in my tank if I think I have an issue and will drain it, purge the lines etc.

[350RRC]

Not a matter of being skeptical as I accept dirt and other stuff may get in through the filler neck. But then shunted along to prefilters or filters. Tanks are generally drained and refilled on a regular basis keeping up a regular cleaning cycle. Algae generally require sunlight to grow, so if present I would think it must come from outside the tank and external contaminate into the fuel - my guess is it would die in petrol or diesel in the dark and not turn into some algal mat inside your tank. I'm not saying that there will not be a case of serious contamination, but it would have to be real case of neglect or stupidity ie the stubbies in the filler neck - someone maliciously putting sand or something else in your tank. Water is the most likely from condensation (should be easily filtered) or flooding in a river crossing (unlikely if your not already leaking fuel, maybe the breather tube, and volume could overwhelm filters).

I was told of a ship full of petrol refined overseas that had to go back and forth over a period of time between Melb and Geelong refineries to unload because its cargo had some 'growth' in it and had to be 'bled' into the Vic distribution network.

DL

[workingonit]

I worked in the oil industry very early on in life. If true then its an example of gross human error or neglect and deceit that such a volume has been contaminated and fed into regular supplies - not from some very rapid act of nature inside the tanker or what ever - and I can't see fuel companies risking their reputation by seeding their regular stock with contaminant. The only time I saw fuel being 'bled' into the system was when a foreman accidentally made a few million litres too much of 2 stroke - he paced all day worried he'd lose his job. Fuel supply and use outside Oz I cannot vouch for - met a naval officer whose job in a fuel blockade was to rope down onto the decks of ships suspected of trafficking oil - you had to watch the swell as you went, not only because of the danger of a hard landing, but that you could actually go through the heavily rusted decks.

As said earlier water in or under the fuel may cause a rust problem in steel tanks. 200 litre aviation drums had to be inspect inside with a torch. Any kind of rust saw the drum get a ding with the steel tip boot. Dinged drums were not for aviation fuel. The dinged drums however were filled with aviation fuel and labelled 'racing fuel' - same, same but differentiated by marketing and safety protocols.

You may get bacterial growth in some circumstances - no light required. Its presence in significant quantities sufficient to cause problems is likely again due to human neglect. It can be a problem in oil cooling systems associated with lathes, owners not regularly clearing the contaminated oil. The bacteria can grow between the sliding faces when tail stocks are not moved for a long time for example, causing rust like pitting.

[cuppabillytea]

JD is quite right in his description of what can and sometimes does, happen. You only need to run an old motorcycle to know that. However modern filtration systems will prevent serious problems most of the time.
Bacteria also can cause serious problems, as stated above. I posted an anecdotal example a while ago.

[sam_d]

Interesting :) Lots of replies but no real consensus.

I can see that air getting into the system is probably the only real issue with letting the fuel get down too low but that's not really a big issue.

The idea that you'll pick up all the crud in the bottom of the tank still doesn't make sense. Even less so since once the fuel is agitated the crud is distributed throughout the fuel so you'll be picking the crud up anyway (albeit in a more diluted form but it'll still get into the fuel system and collected by the filter).

An old wives tale based on something from early motoring that has been passed down perhaps?

As it is, I rarely go below 1/4 tank when i'm out and about anyway. I maintain as full a tank as possible when I'm parked up between trips to reduce the available space for oxygen/moisture getting in and causing problems though for the reasons described in above post.

[rangieman]

Thing is all these modern cars with in tank Fuel Pumps run swirl pots in the base of the pump [wink11]
If you run your tank low and i have been guilty of this once or twice you should not have a problem but if you consistently run low and are consistently picking up **** or water you have picked up a bad batch of fuel once or more often so change your supply source .
I have run mine down to when the fuel light comes on few times and am not concerned .
Yes i drain my filter regularly as well as change fuel filters regularly and never notice anything out of the ordinary to be concerned about[wink11]

[Mick_Marsh]

When things are cold, things tend to get smaller, contract. You're a bloke. You shouldn't need to be told that.

Interestingly, things contract at different rates. i.e. a fuel tank doesn't shrink as quickly as the contents. When it gets cool, the fuel tank sucks in air, or rather, the higher air pressure outside forces damp air inside the fuel tank into the vacuum caused by the contents of the tank getting smaller at a quicker rate than the tank.

Another thing, cool air is usually quite damp, or rather, air is usually quite damp. When this air gets into the tank, the moisture tends to condense from vapour to liquid inside the tank. The drier air then tends to escape from the tank as the temperature/pressure rises leaving the moisture in the tank.

Now, if there is little fuel in the tank, lots of moisture laden air gets into the tank. If there is lots of fuel in the tank, less moisture laden air gets into the tank. This is because air contracts at a greater rate than the fuel.

If you get moisture in your fuel tank, you can get all sorts of problems i.e. metal fuel tanks rusting and leaking or rust flakes clogging fuel systems.

This is why I try to keep my fuel tanks full of fuel.

[JDNSW]

Yes. More important if you live on the coast.

[Gordie]

For those who doubt algae can flourish in diesel...I had a Detroit powered truck in NZ...would be driving along and just die on me...filter would be full of black algae "diesel bug". Lives in the interface between water and diesel. Got myself a big racor pre-filter, and periodically treated my tank with a chemical for diesel bug. Never had a problem after that. It does exist.

[rangieman]

For those who doubt algae can flourish in diesel...I had a Detroit powered truck in NZ...would be driving along and just die on me...filter would be full of black algae "diesel bug". Lives in the interface between water and diesel. Got myself a big racor pre-filter, and periodically treated my tank with a chemical for diesel bug. Never had a problem after that. It does exist.
Im don`t doubt diesel algae [wink11] This Thread is more on about the wife`s tale [bigwhistle]

[Gordie]

Im don`t doubt diesel algae [wink11] This Thread is more on about the wife`s tale [bigwhistle]Ahhh true...had a re-read and you are right! Oh well...back on subject re this thread...don't know about wives tale or not.....but my daughter had a getz...whenever she ran the tank down low, it would start running rough, put some E10 in there and it would run ok again. Waggle my finger at her and tell her not to run it down so low...until next time when she did the same, and go through the process again...So I could only conclude that there was indeed water or crap getting sucked in when it got low!!

[cuppabillytea]

E 10 would have the advantage of taking small amounts of water into solution, keeping the tank relatively water free, wouldn't it?

[Gordie]

E 10 would have the advantage of taking small amounts of water into solution, keeping the tank relatively water free, wouldn't it?Yes, cheaper to fill up with E10, than to pour in a bottle of methylated spirits. Same result. And it did the job.

[Mick_Marsh]

E 10 would have the advantage of taking small amounts of water into solution, keeping the tank relatively water free, wouldn't it?
That's the theory. I wonder if it works in practice?

As the Commodore got older, whenever I ran the tank empty, it would destroy the in tank fuel pump.

The Merc has a swirl pot in the tank. It was old, well, it's a 1965 Merc and I suspect the original tank. It was very rusty in the tank. The holes into the swirl pot were partially blocked and restricted the flow of fuel into the swirl pot. When the tank was full, no problems. Under half full it would only run for 20km or so before I had to sit on the side of the road for fifteen minutes whilst the fuel seeped into the swirl pot. It's all fixed now and I try to keep the tank full to prevent the new tank from rusting up.

I keep one of the 101s tank full. One of the others only had a splash in it. It rusted through. I have a stainless tank for it now.

[350RRC]

That's the theory. I wonder if it works in practice?

As the Commodore got older, whenever I ran the tank empty, it would destroy the in tank fuel pump.

The Merc has a swirl pot in the tank. It was old, well, it's a 1965 Merc and I suspect the original tank. It was very rusty in the tank. The holes into the swirl pot were partially blocked and restricted the flow of fuel into the swirl pot. When the tank was full, no problems. Under half full it would only run for 20km or so before I had to sit on the side of the road for fifteen minutes whilst the fuel seeped into the swirl pot. It's all fixed now and I try to keep the tank full to prevent the new tank from rusting up.

I keep one of the 101s tank full. One of the others only had a splash in it. It rusted through. I have a stainless tank for it now.

My first car was a VW wagon with the flat 4. It was always getting water in the main jet (or jets, can't remember if this was after a twin carb conversion). The jet(s) were in the bottom of the float bowl.

Major PITA to dismantle and clean. The tank got a thorough clean more than once.

Someone told me about the metho trick and how it works.................... no more probs after that.

DL

[DiscoMick]

Next time you replace your fuel filter rip the old one open and look at what is inside.

I thought this was an interesting discussion of algae in boat fuel tanks. It recommends keeping the tank full when the boat is stored for long periods.

Preventing and removing diesel algae - BoatCoachBob (http://www.boatcoachbob.com/articles-boat-maintenance/engines-inboard/prevent-remove-diesel-algae/)

I suppose we could just embrace our algae and produce algae biodiesel, but we'd have to become experts in transesterification, which sounds painful.

How Algae Biodiesel Works | HowStuffWorks (http://science.howstuffworks.com/environmental/green-science/algae-biodiesel.htm)