I always try to avoid running my car down that far on fuel, as I reckon the concentration of particles (rust/sludge/whatever) per litre of fuel gets very high at that point. 

I'd take a guess at either rust and dirt in the bottom of the tank getting swirled around more when the level is low, and partly blocking the pickup OR water in the tank.  Off the top of my head I can't think whether petrol is lighter or heavier than water.  I suspect its lighter, which then wouldn't really make it a viable theory.

Either way, to fix it I'd start by removing the fuel tank, draining it completely, sending it off to a radiator place (or similar) to be flushed, and then use compressed air to blow back down (from engine) the fuel supply line while its disconnected from the tank.  Fill with 100% fresh fuel, and see what happens - also leave that Spitfire additive out for a while.  Some of those additives seem to cause more harm than good.

Alternatively, don't let it get down that far.  I fill my cars up Wednesday mornings (cheapest fuel), regardless of how much is in it.  If I drive my Rangie all week, it uses half a tank, and the Alfa might use a little more than half, but thats as low as they get.  (Although part of the reason for that is both are EFI cars with carby fuel tanks and no surge tank  )

Guys
All of the above advice is good, rule 1 for all fuel injected cars is never run the tank below 75% empty from there on it picks up the solids in the bottom of the tank, while most of the time these will clear from the injectors, sooner or later something will not and the opitions are replace the injectors or service then, (take a pile of money and hope). Yes feul is lighter then water, 1/2 cup of metho will remove this, this also helps to stop the tank rusting out, 116's are prone to this. For some reason the Classic Alfa V6 runs better on the BP top end fuel, don't ask me why but it is true and I don't know about the current V6 motors. Shell Optimax/Vpower seems to be a bit varible, I know that when Shell Aust up graded there plant that the sucessfull Japaness contractor C____ed up the design of the Cracking towers and this effected the top end products such as Optimax, I know this was a ongoing dispute, but don't know if the these problems in the plant where ever resolved 100%. Rremember drive your V6 hard but treat them with TLC as you own the best sounding motor ever, when Top Gear refer to the classic Italian V6 sound they mean a GTV6 voted the best sounding engine ever by a panel of top motor writer in europe.

Organic Chemistry 101

As an organic chemist, I can tell you water is not lighter than petrol (hydrocarbon).  Given the density of water is 0.998 to 1 g/mL, petrol is less than that, about 0.7-0.8 g/mL, so petrol floats on water.  Here is your chemistry lesson, do this experiment, take a bottle, add some water, then add either petrol or vegetable oil, where is the water layer?  I do this type of experiment pretty much every day.

Metho or methylated spirit which is ethanol with methanol and a fouling agent (to prevent you from drinking it) added, is miscible with water, ie metho dissolves or mixes with water, such as vodka.  So putting metho in your petrol tank will take out the water condensed in there.  At low concentration, metho dissolves water, the metho or ethanol is soluble in petrol (at low concentration), so the tri-mixture is all miscible together.  Basically when you add in metho, you have made up E5 or E10!!!

Neil

This was my initial suspicion, and I was a little surprised to hear otherwise, but didn't have the energy to go and look myself - I do know from experience that diesel is lighter than water.... and come to think of it, I'm pretty sure I've seen petrol floating on diesel.... hmm - I hate when I discover I already knew an answer.


Excellent explanation - thank you Neil!

So, how did it go with the orange juice?  I reckon raspberry cordial is more effective myself.

1.  You have a problem.

2.   Fix the problem rather than come up with a variety of ways to work around it 

I'd begin by cleaning out the tank.  I suspect you have a buildup of crud in the tank, and the fuel cleaner has probably made it worse by dislodging years of buildup.   Or there may be a bit of plastic or something loose inside the tank that only blockes the pickup with low fuel.  BTW, the metho trick will only remove so much water.  Above a certain concentration the alcohol won't mix with any more water so you will still get free water in the tank.

I suspect the real issue is a tired fuel pump.  The fuel pipe in the tank is actually a standpipe and the pump has to draw fuel up the pipe.  With a fuller tank it's easy, but with a low tank the pump may be struggling.  Check it out now so you don't end up stranded (with a full tank  )