Should I be Leaving a quarter tank?

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jscottnjs
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Should I be Leaving a quarter tank?

Unread post by jscottnjs » January 28th, 2018, 7:44 pm

Hi guys,
I have just purchased my first 4wd its a 2004 Holden Rodeo 3.6 v6. The car has dual system petrol and gas, in the owners manual the previous owner has made a note to leave a quarter tank of petrol at all times, something to do with the dual fuel.
Does this sound correct or can I run both tanks empty as a normal vehicle, I have never had a gas vehicle either.

Peter Aawen
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Re: Should I be Leaving a quarter tank?

Unread post by Peter Aawen » January 29th, 2018, 1:29 am

Hi jscott, welcome to the Forum! :thumb:

That previous owner was a smart bloke, cos even tho it's smart to leave enough petrol in the tank of a dual fuel vehicle anyway, cos it's probably easier to start on petrol if it's hot or the converter has iced up, besides which if you let the tank run dry then any seals & gaskets will probably dry out & degrade over time.... but also, ANY modern 'normal vehicle' (that runs a fuel injection system, be it ULP, LPG, or diesel powered) should always have AT LEAST 5 or so litres of fuel in the tank, any tank, or risk damaging the fuel pump &/or fuel injection system!! So leaving a quarter tank of fuel in there is a smart idea for a number of reasons!! And you should do the same with any and every EFI or Fuel Injection engined car too, regardless of whether it runs on LPG or ULP or diesel!! Weren't you already doing that?? Check the glovebox manual, it probably says to refuel once the gauge gets to about 1/4 or the fuel light comes on, that's usually been in there somewhere in almost every glovebox manual for a fuel injected car since about 1974!! :o

This is because the modern fuel pump & injection systems use the fuel remaining in whichever tank they are running off as both lubricant & cooling medium for the fuel pump & the entire injection system, and it takes at least about 5 or so litres of fuel remaining in the tank to do that properly - any less and the very fine tolerances in the fuel system may start to be compromised by heat because there's not enough fuel left in the tank to dissapate the generated heat properly at the same time as maintaining lubricating capability too!! And it doesn't help any that the filter/s designed to keep large contaminants out will also be beginning to choke up on the concentrated debris & contaminants that generally collects in the bottom of fuel tanks either!!

Now sure, running the level lower than that probably isn't going to kill the fuel system immediately or instantaneously it happens the first time, but these modern systems are made to very fine tolerances and levels of efficiency, and just one molecule of water running thru a system that has already started to over-heat will likely be compressed massively between two of the rotating or bearing surfaces if it happens to get there (remember that all these contaminants get concentrated in the bottom of the tank, so the chances of this occurring have already been increased significantly!!) And when that molecule, that was so small it went straight thru the filter (& normally would've just been burnt off in the combustion chamber, but the fuel pump that is always running at high pressures has now reached temps of well over 100 deg C cos there isn't enough cooling going on!!) gets pressurised as it's squeezed thru the pump or injector, it immediately flashes off to super heated steam because of the pressure & the now raised temps, and in doing so it can melt a tiny molecule of metal on each of the surfaces it's compressed between, welding them together in that moment so that as the surfaces rotate on or part, that little spot of hardened metal will rip out of one side leaving a hole there & adhere to the other side leaving a lump of welded hardened metal there!! Yeah, the hole & the lump are both microscopically small, but still, next time they come around as the pump/injection system keeps operating, they don't necessarily come together & neatly fit into each other, the hole will probably be one of agrowing number of slightly lower spots of degraded pressure in the flow of pumped fuel, & the growing number of lumps of hardened metal will start to rip the crap out of whatever bits of other material they hit, damaging all that too & compounding the microscooic degradation that will now occur with every single rotation of the pump/operation of the injection system &/or passage of fuel!! And that goes on with an increasing number of microscopic molecules on every revolution of the pump multiplied by millions of molecules for all the time the pump/injection system runs!! So it'll likely take time, maybe even tens of thousands of kilometres of injection system or fuel pump running time, but it WILL happen irrevocably and cumulatively, slowly ripping the guts out of that system's ability to run at peak performance and slowly but surely degrading the system's & the engine's performance & fuel economy!! And the more often the fuel level gets too low for it to properly cool & lubricate the pump & system, the more microscopic damage will occur so that it'll slowly but surely increase the damage & get worse with time & kms/engine run time anyway.....

Or if there's enough dodgy fuel or even just partially dissolved/suspended water molecules floating around in the fuel gathered & concentrated in the less than 1/4 of a tank dregs left in your tank/tanks; or if the filters are a bit choked already cos they are overdue for swapping out since you (or the previous owner) missed that last service; or any one of a myriad of other possibilities; ESPECIALLY in a 4WD that possibly gets fed marginally contaminated fuel a bit more often than the average car on the road; or EVEN MORE ESPECIALLY if you make a habit of running your fuel level lower than the 1/4 of a tank or whatever is recommended in the particular glovebox manual, then it might just be the very next rotation of the fuel pump or engine that is the very last one before all that cumulative & irreversible damage finally gets to be too much for the EFI system to operate or the engine to continue to run!! And that might happen sooner rather than later or possibly even RIGHT NOW!! Do you really want to risk that??

So jscott, it sounds like you've done well to get a car that has had a previous owner who's been a tad more aware than most, and one who's very likely to have kept the fuel level above the critical low limit and kept therefore the fuel pump & injection system in just a little better condition than the average owner or anyone who never bothered to pay attention to the warning about keeping the fuel level remaining in the tank up above 1/4 or so, or at least higher than you could fairly safely run old carby fed engines!! He might've been doing it so he didn't let the seals & gaskets in the petrol system dry out while it ran on gas, or maybe he was aware that you should keep the level above about 1/4 anyway, for the reasons above on ANY modern Injection engined car, but it IS better for the fuel & injection system & it seems he at least tried!! Sooo, Keep on doing that & you might be lucky enough to preserve the fuel pump & injection system for longer & in better condition/operational capacity than most! And that can only be a good thing! Good Luck! ;)
An Ex-Service person is someone who thought enough about their country & how great it is, how lucky we are to live here, to write a blank cheque made out to 'The People and Commonwealth of Australia' for the value of 'Up to & including my Life!'

jscottnjs
I'm new, be nice!
Posts: 5
Joined: January 26th, 2018, 11:01 am

Re: Should I be Leaving a quarter tank?

Unread post by jscottnjs » January 29th, 2018, 9:10 am

Thank you very much, I will keep on filling once i get to 1/4 tank

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