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Petrol In Your Tank

rob boban

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Hi raiders,

just wondering if some more expierienced boaters could help me. I was reading an article about boat motors and was a bit concerned when i read that fuel sitting in your tank for long periods at a time (2 -3 months) is actually not good for your motor, something to do with the fuel seperating. Any info would be appreciated.

Cheers, :beersmile:

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Hey Rob are you talking about phase seperation? "This occurs when E10 or ethanol blended fuel absorbs moisture causing the ethanol to seperate from the gasoline. Moist air will cool to form condensation in every tank so there is a ready supply there. Keeping the tank full helps reduce the problem but may lead to a greater amount being contaminated. You cant remix it, you cant shake it, you cant put in additives to cure it.

After seperation we have low octane fuel at the top, causing preignition or pinging. When enough ethanol reaches the fuel pick up a dose of 100% solvent runs through a system capable of only taking 10%." Thats what i read.

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Firstly , NEVER run E10 in outboards .

Secondly , Petrol , as you would be aware , is refined from a heavy crude oil . During the refining process , the molecular structure is changed , and what we end up with is motor spirit (petrol). There are volatile substances in this mix , which tend to evaporate.Old, unused, petrol goes 'stale', smells foul, and rots tanks: the lightest fractions can evaporate whilst the remains gradually join their molecules back into heavier stuff like that they were once cracked from. This is the "gum" that befouls so many outboards that have been unused for a period of time.

Direct injection motors have one advantage in that with a totally closed fuel system no oxygen exists and no sunlight so if your motor is stored on a constant cool temperature the fuel will remain OK for a reasonable time for storage rather than fractionalise / seperate oxidise , evaporate and leave gume and minerals.

Ross ( with thanks to Pelican for a previous post on this issue ) .

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So if ive got a 20l jerry can, (fully sealed) with 20l of 98 optimax unleaded from last christmas, is it ok to use in old 85hp merc 2stroke? (the fuels been mixed with oil already)

p.s never realised its been that long since taking the boat out..almost 10 months :(

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Optimax has had some storage issues. 2 stroke oil in general shortens petrol life even with additives in the newer synthetic oils. Oil companies say between 2-3 months for fuel storage generally in ideal conditions if you want to check just send Shell Australia a quick email.

Now how can I put this nicely - expensive engine that stops you getting caught with a damaged or broken down motor versus $50 in fuel. New tech old tech , 2 stroke , 4 stroke all can get stuffed by bad fuel and it ain't worth it. If your car breaks down you don't drown.

Being the cheap mongrel I am I would just run it through a older car with 5l to a 50 plus litre tank and only mix up fresh fuel for the boat. Check your old boat tank for oil sludge as well as it comes out of solution. You can never freshen old fuel by mixing in fresh fuel and I would never do it in a marine environment. Marine motors are high revving performance motors under stress with small water galleries so will burn pistons quickly not like car engines which have knock, exhaust and inlet sensors. The better thing to do would be to dispose of it at the council collection days where they do sump oil etc in 5l lots.

Octane boosters etc etc do not refresh fuel. The only thing that does if running it back through the refinery

We general have 150l of fuel around the place in jerry's for genny and boat etc and we simply have a zip tie with a XXml premeasured oil bottle attached to the top of the boat jerry for and only mix it if we need it. That way it can go in the car , the aux on the boat , tinny or the genny so we always are rotating and have fresh fuel.

I think plastic Jerry cans don't store fuel as well as good quality metal ones but that is a personal thing.

As for the boat tank empty / full thing it depends on a couple of factors. fuel stores best with no air contact as it oxidises ( part of going stale) so minimal air contact is best - ie full tank. A sealed tank is best but boat tanks have breathers if they are a underfloor. A sealed tank stops the volatiles / aromatics from evaporating from the fuel. In larger under floor tanks many had an issue with condensation in them so the theory was keep em full, no air no condensation but that was only good management if you used the fuel quickly. Water in petrol sends it stale faster. Other one is that full tanks don't explode where as a empty tank full of vapour could. A lot of that came from diesel boats where water is a real issue in tanks as it allowed bacteria to grow in water in the fuel and clogged filters and injectors. They now have better biocides to treat diesel with.

If you ever need to store fuel for an extended period for a farm etc there are additives you can ask the refinery about that preserve it's qualities. Each fuel company has their formulae and it is often used in drum fuel for remote communities.

Best thing with fuel is to buy it fresh, keep it in clean tanks, use it fast, always have 2 separate sources and have a good filter with water separator to protect your motor. I prefer to keep tanks empty and fill with fresh fuel regularly and have suitable water separator if I get condensation in the tank. If I know I am not using fuel for some time I will use a fuel additive suggested by the refiner we are using.

Now some old codgers are going to say all the above is crap - well good on em. The next one is the story about the jeeps on the islands 20 years after WW2 that with a new battery started on 20 yo fuel ( its true ) Well old codgers it was the old leaded mill spec fuel with additives. All old leaded fuels stored well and held their octane gums and waxes well and to put it simply new fuels don't and have a lot of trouble holding octane.

Last one - Ethanol - never ever on a boat in a tank and I don't care if a manufacturer says their new motor runs OK on it and hull manufacturers say their fuel tanks were built for it. It will absorb moisture,it can drop that water in tanks, it will erode motor parts, it will cost you more in servicing, your motor will not produce full Hp ,can rot even new rubber parts and is not cost effective for octane level. Ethanol fuel can form an corrosive acid if it absorbs water and settles in a tank and it can't just be mixed up again, gums and waxes can also come out of solution

Ethanol is basically metho- I won't drink it and I don't expect my boat motor to either.

Sorry for the long post but it is probably a good reminder to us all as I am noticing more servos with sneaky labels on which fuel does and does not have ethanol in it since last year.


Edited by pelican
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Great post Pel

Great Topic at this time of year after lots of boats

have been in storage.

It reminds me to drain mine it hasn,t been done for a while

Yeah - that time of year where it is good to check all fuel fittings and hoses so they are good for a trouble free safe season. I've gotta check dates on my safety gear as well- normally have it written on the list on the container but it has come off. I also run through all electrical connections and make sure there is no corrosion and everything is clean and working.

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Sorry for the long post but it is probably a good reminder to us all as I am noticing more servos with sneaky labels on which fuel does and does not have ethanol in it since last year.

I nearly got caught out with that. The mobil on the corner of west botany st and bestic st, just up the road from the cooks river ramp at kyeemah only sells premium and e10. :ranting2: Had to all the way down to the shell on grand parade. After every trip out I drain the fuel out of the boat into jerry cans then put it into the car.


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