Question about old fuel still in the tank when I fill up

[back2boats]

I have read a few threads about aging of fuel, and the sort of times mentioned seem to be about a month or two, after which fuel might be a bit too old to use.

OK, so I have recently bought a small tinny with an small outboard that came with a 24 litre tank. It seems to me it will go forever on this tank, it uses so little fuel on a typical (for me) fishing trip. So I guess I will just keep topping it up every few trips, and I expect to be using it about once a week. But that means that there will always be some old fuel in the tank, and even though it is mixed up every time, at the 2 month point, for example, there will be SOME of the original fuel in there.

So, does mixing it with fresh fuel at each top up cure the problem, or should I throw out the remainder every couple of months and start fresh?

[juggernaut]

If it starts it's fine. I had a project car sitting for 2 years and started no probs with old fuel.

If its a 2 stroke with pre-mix might need to add stabil or similar.

[AndrewB]

Talk to any outboard mechanic and 70% of motor issues are stale fuel

[Moonlighter]

My thoughts as follows:

Generally, you can smell fuel that has gone bad, it smells off, kind of rotten. And, once it has gone off, it is off, adding fresh fuel to off fuel is like adding fresh milk to sour, you just get a bigger lot of sour.

Half empty tanks seem to go off quicker than full-to-the-top ones.

Also, full tanks that are kept in a garage, sealed closed, out of the sun, and away from hot places will last better than those that aren't.

Finally, If you have any doubt, dont use it. Dispose of it, or if you are just being careful, syphon it into your car or use it for the mower.

Maybe consider getting a smaller tank for the boat if you only ever use a small quantity. You can always take a second 5 or 10 litres in a can with you and some oil to mix it with as a backup.

[back2boats]

If it starts it's fine. I had a project car sitting for 2 years and started no probs with old fuel.

If its a 2 stroke with pre-mix might need to add stabil or similar.

It is 2 stroke. How long will it last with stabilizer added?

[back2boats]

Talk to any outboard mechanic and 70% of motor issues are stale fuel


Yep. I'd rather waste a little fuel then have motor problems, especially out on the water. :)

[back2boats]

My thoughts as follows:

Generally, you can smell fuel that has gone bad, it smells off, kind of rotten. And, once it has gone off, it is off, adding fresh fuel to off fuel is like adding fresh milk to sour, you just get a bigger lot of sour.

Half empty tanks seem to go off quicker than full-to-the-top ones.

Also, full tanks that are kept in a garage, sealed closed, out of the sun, and away from hot places will last better than those that aren't.

Finally, If you have any doubt, dont use it. Dispose of it, or if you are just being careful, syphon it into your car or use it for the mower.

Maybe consider getting a smaller tank for the boat if you only ever use a small quantity. You can always take a second 5 or 10 litres in a can with you and some oil to mix it with as a backup.

I have never smelt bad fuel, but if, as you say, it's obviously smelling rotten, I won't be using it.

It will be mostly sitting in the garage, out of the sun, and sealed, in as much as I tighten down the vent screw on the tank.

I will monitor my usage, and consider the smaller tank option. That sounds like a good idea if I don't use it all in a month or two. Thanks

[juggernaut]

It is 2 stroke. How long will it last with stabilizer added?

I can't be definitive as I have only used in my pre-mix garden equip. Having said that I can get a year out of pre-mix fuel without stabiliser but it is in a sealed container.

Most of the problems when using pre-mix is not the fuel in the tank but the oil gumming up the carbys /fuel system when fuel evaporates out of the carby's/fuel system leaving the oil residue. It is for this reason that most mechanics i talk to suggest starting a 2 stroke outboard every two weeks or least every month. Any longer and your better off running it dry.

[back2boats]

I can't be definitive as I have only used in my pre-mix garden equip. Having said that I can get a year out of pre-mix fuel without stabiliser but it is in a sealed container.

Most of the problems when using pre-mix is not the fuel in the tank but the oil gumming up the carbys /fuel system when fuel evaporates out of the carby's/fuel system leaving the oil residue. It is for this reason that most mechanics i talk to suggest starting a 2 stroke outboard every two weeks or least every month. Any longer and your better off running it dry.

Sounds like the stabiliser will make it last long enough for my purposes.

The mechanic who serviced my motor recommends running it dry each time anyway. Easy enough to do as part of the flushing routine.

[Fed]

What motor is it?

[back2boats]

What motor is it?

Early 2000'ish 8HP Yamaha

[Noelm]

Never been a huge fan of stabilizers and stuff like that, far better to get a smaller (11l) tank and just keep the fuel fresh, additives tend to give you a false sense of security I reckon, mind you, I also think people are getting super paranoid about fuel going off, I do agree that adding new fuel to old rubbish fuel just gives you a lot more rubbish fuel to deal with.

[newchum]

I had an old evinrude pre 90's sat under a lean too for 18 months with 60 litres of premix fuel. Changed filter primed carbs and started after about three revolutions, idled out of harbour, ran engine at 5000 revs for 15 miles did not miss a beat. One thing I always did was run the carbs out before parking the boat. Not a big believer in stale fuel being blamed for engine problems (not that fuel does not degrade over time ) but dirty carbs are a bigger problem.

[back2boats]

I also think people are getting super paranoid about fuel going off

Well I have been told that it does go off faster now than the old leaded fuel used to. Have no idea if it's true or not though, but it would explain why people have more concerns now.

[fisho64]

the ethanol fuels that seem common in Queensland are likely the biggest danger.
They are prone to what is variously called "phase separation" amongst other things.
Because mineral fuels are not water soluble but ethanol (and biodiesel for that matter) are soluble in both mineral fuel and water, when condensation occurs (half full tank!)
the small amount of water is absorbed by the ethanol but at a certain level the ethanol/h2o mix can no longer stay in suspension and the ethanol drops to the bottom.
From there your outboard tries to run on 100% ethanol.

[dnej]

I called Shell some time back in regards to this issue. 45 days is the recommendation, and the further north you go, the more it changes, as they change the recipe, so to speak.
Fuel is cheaper than pistons, and bores.
One mechanic had 13 motors in his work shop. He maintained they were all stale fuel issues.
And don't use it in your motor mower, as its not the same two stroke oil.
David

[back2boats]

the ethanol fuels that seem common in Queensland are likely the biggest danger.
They are prone to what is variously called "phase separation" amongst other things.
Because mineral fuels are not water soluble but ethanol (and biodiesel for that matter) are soluble in both mineral fuel and water, when condensation occurs (half full tank!)
the small amount of water is absorbed by the ethanol but at a certain level the ethanol/h2o mix can no longer stay in suspension and the ethanol drops to the bottom.
From there your outboard tries to run on 100% ethanol.

Certainly going to avoid ethanol then.

[back2boats]

I called Shell some time back in regards to this issue. 45 days is the recommendation, and the further north you go, the more it changes, as they change the recipe, so to speak.
Fuel is cheaper than pistons, and bores.
One mechanic had 13 motors in his work shop. He maintained they were all stale fuel issues.
And don't use it in your motor mower, as its not the same two stroke oil.
David


45 days fits with other stuff I have read too, and yes, a little wasted fuel is certainly less expensive than repairs.

[rexaway123]

some good info for you straight from the fuel makers!

http://www.bp.com/content/dam/bp-country/en_au/media/fuel-news/petrol-life-vehicle-tanks.pdf

[lifestyle]

Just put the old fuel in your car when it is 3/4 full. It will get diluted down I have done this for years and never had a problem with t he car or boat.

[back2boats]

some good info for you straight from the fuel makers!

http://www.bp.com/content/dam/bp-country/en_au/media/fuel-news/petrol-life-vehicle-tanks.pdf

I knew my high school chemistry would be of use some day. :)

Great info. A standout is the advice to top up if not used for a week when it is sitting in the tank of the equipment. This is because of the loss of volatiles, which makes sense. I suspect an outboard tank is better sealed than some, but still..

[back2boats]

Just put the old fuel in your car when it is 3/4 full. It will get diluted down I have done this for years and never had a problem with t he car or boat.

I have heard this a few times, yes.

[juggernaut]

I knew my high school chemistry would be of use some day. :)

Great info. A standout is the advice to top up if not used for a week when it is sitting in the tank of the equipment. This is because of the loss of volatiles, which makes sense. I suspect an outboard tank is better sealed than some, but still..

Topping up fuel was only 1 of the 4 recommendations in a strategy for intermittently used equipment, also

Keep the tank full where possible to avoid condensation
Hotter plug
Better fuel

I've only used BP98 in all my cars, boats and equip. Given you are adding oil to fuel (which also lowers the octane rating of fuel and is more likely to cause detonation and pinging) you'd be better off with BP98 or better to assist in preventing engine damage from detonation and pinging compared to 91 or 93 octane rated fuel with oil.

Ethanol fuels also require a richer mixture so unless you are going to rejet the carbs to consistently run on ethanol, you wouldn't touch ethanol based fuels. For example E85 requires approx 30% more fuel compared to BP98. Some cars these days have flex fuel sensors which senses the ethanol content in the fuel and the ECU compensates for this.

[back2boats]

Topping up fuel was only 1 of the 4 recommendations in a strategy for intermittently used equipment, also

Keep the tank full where possible to avoid condensation
Hotter plug
Better fuel

I've only used BP98 in all my cars, boats and equip. Given you are adding oil to fuel (which also lowers the octane rating of fuel and is more likely to cause detonation and pinging) you'd be better off with BP98 or better to assist in preventing engine damage from detonation and pinging compared to 91 or 93 octane rated fuel with oil.

Ethanol fuels also require a richer mixture so unless you are going to rejet the carbs to consistently run on ethanol, you wouldn't touch ethanol based fuels. For example E85 requires approx 30% more fuel compared to BP98. Some cars these days have flex fuel sensors which senses the ethanol content in the fuel and the ECU compensates for this.


Yes I read all of it, some of which I was already doing.

Like keeping the tank full and using better fuel with higher octane to begin with, and avoiding ethanol is a given as the manufacturer's manual says don't use it.

[tunaticer]

My boat has sat idle for well over 6 months esp in it's early years.......it is having a 19th birthday next month and never had a drama with stale fuel. The motor would be well in excess of 3000 hours now and the compression is still good.
Not a bad run for two sets of spark plugs and 18 impellors plus about 7 new props I reckon.

Oh and a full set of rubber engine mounts....seems the dislike hitting reefs just like my prop does.

Go the dirty Rude!!

[tunaticer]

My boat has sat idle for well over 6 months esp in it's early years.......it is having a 19th birthday next month and never had a drama with stale fuel. The motor would be well in excess of 3000 hours now and the compression is still good.
Not a bad run for two sets of spark plugs and 18 impellors plus about 7 new props I reckon.
Go the dirty Rude!!

[Crocodile]

Hello back2boats,

I used to have the same issue when I had an 8hp.
I would carry a five litre jerry of mixed 50:01 as a reserve.
Five litres goes a long way with 8 hp.
When the main tank ran low I would transfer the 5 litres into it and then re-fill and re-mix the 5 litre jerry.
I would touch the jerry to the main to dissipate any static electricity before I started pouring.
I was simple to do and I never had any problems.

[back2boats]

Hello back2boats,

I used to have the same issue when I had an 8hp.
I would carry a five litre jerry of mixed 50:01 as a reserve.
Five litres goes a long way with 8 hp.
When the main tank ran low I would transfer the 5 litres into it and then re-fill and re-mix the 5 litre jerry.
I would touch the jerry to the main to dissipate any static electricity before I started pouring.
I was simple to do and I never had any problems.


5 litres certainly will go a long way. I was out all afternoon today, and I can't see that the needle moved much at all on the 24 litre tank :) I do have a 5 litre jerry can for the car that I could use as you did. Thanks for the tip on how you did it.