keep gas in tank or empty?

[mrpike1973]

i know many ideas here but with my boat motor is it best to run stabilizer like blue stabil in the tank. keep it full and leave it for the winter. or is it best to run them dry. the motors are 9.8 mercury and 15 hp evinrude thanks

[reinhard1]

we have had threads like this before with different views. i'm not a expert on this but this is what i do and have done since i bought my boat/motor in 2001. i have a 115 hp 4stroke Merc. i put only pure premium non oxy in my tank. i topped it off this year but last year i know it wasn't completly full. i do put one can of Sea Foam in the tank each fall when i put it away. my motor runs perfect now and has since i bought it. i only changed spark plugs one time even though it ran fine. good luck.

[EBass]

Run stabill and keep it full. The more gas the less water vapor.

[Kyhl]

If it is a plastic tank, I don't think it matters.

If it is a steel tank, fill it up.

As mentioned, empty tanks have open side walls that are available to collect condensation. If it is a steel tank then that condensation can create rust inside the tank.

I've not had a problem with plastic tanks and condensation when filling with fresh gas and some seafoam in the spring.

[delcecchi]

The advantage of not being full in the fall is you can put in some fresh gas in the spring. It may or may not matter to your motor, but apparently the recommendations have changed recently.

[BoxMN]

With my 25p evinrude and 6 gallon plastic tank, I always ran only non-oxy gas and a light capful or so of seafoam in every tank, and drained the tank for winter. Never ran the carbs dry that I recall on purpose. Each spring, new non-oxy gas with a capful of seafoam, and it started like a Mercury meaning on one turn of the key, hehe!

In our inboard tanks, we just leave it with whatever is in there, but we run a little seafoam and non-oxy gas (usually) all year long. Spring it just gets fresh gas and off we go. Good luck, I know others do different, but this works for us over three different outboards of various ages and hp's.

[fishnowworknever]

If you are using regular gas with ethanol it will collect water after some time..

Older motors were not designed for E10 gas and the water it will absorb.

[dan z]

Fill er up, run some stabill or sea foam for a min and call er good

[mrpike1973]

thanks for the replies. i have plastic tanks run non oxy all the time. i think i will leave whats in the tanks and leave it. is it ok to have a half full tank or will i still get condensation thanks

[Kyhl]

You will get some condensation with a half full tank. Treat it in the fall and fill it with fresh gas in the spring and you should be fine. That is what I do.

The way I look at it, the exposed surface area inside the tank is not that much different between a full and half full tank. Unless a full tank is filled up into the filler tube, the top of the tank will be exposed to air, allowing condensation. Therefore the difference in exposed surface area between a full and half full is the surface area of the tank sides, which is usually less than the surface area of the top of the tank.

Assuming most tanks are longer and wider than they are tall.

[delcecchi]

If you close the vent on a plastic portable tank, no air goes in and out so you get no condensation. Built in tanks don't have closeable vents, so they can get condensation. But the air in winter in Minnesota is extremely dry so I am skeptical about how big a problem it is. I would think it would be much worse in summer.

[Kyhl]

Hi Del,

It's not the Winter that is a problem. It's the Spring and Fall when temps are above freezing and raise then drop back down over night. Each heating and cooling above the freezing point will cause condensation on the exposed surface areas of the tank, inside and out.

I agree, in the Winter when temps are consistantly below freezing will not create much condensation.

[delcecchi]

Temps go up and down year around, and there is often heavy dew even in summer. So isn't condensation a year around phenomenon?

What happens to the water resulting if my boat sits with a half tank for a few weeks? And how much water are we talking about?

I am still unconvinced that condensation on the inside of a built in gas tank is much of a problem. I recall that my marine mechanic who used to recommend filling the tank in the fall switched to telling me to do it in the spring. Apparently stale gas was a bigger problem.

What is the recommendation of the Motor companies? If anyone would know they would.

I did put on a cartridge type filter to be safe, I hope.

[Kyhl]

That is why I started by saying,

If it is a plastic tank, I don't think it matters.

If it is a steel tank, fill it up. {so it doesn't rust}

Because I really don't think condensation is an issue with treated gas. Treat whatever gas is left and you should be good to go.

[delcecchi]

That's what I do.

[kerryd15]

I figure the biggest water problem would be if you ran it hard for a day in the middle of summer at 90 degrees and 80% humidity. Then left the tank half or less full assuming you have a 20 gallon tank and use 10 out that day, then park for winter. You now have 10 "gallons" of very wet air in the tank. What happens when we have frost? Or gets to the dew point? Cold air can't hold moisture. I think it would make a big difference if you used the same 10 gallons of gas on a warm dry fall day with 20-30% humidity.

[delcecchi]

I looked it up. At 86 degrees, saturated air holds 30 grams/cubic meter or about .12 grams/gallon (a cubic meter is 264 gallons). So your 10 gallons of air has 1.2 grams of water. A tsp of water is 5 grams, if I remember correctly. So the 10 gallons of air has 1/4 tsp of water in it.

At least I hope I did the conversions etc correctly. One shot doesn't look like a problem.