Gas Tanks – Full or Empty?

Dear Dr. Motorhead,

I am responding to the article “Laying Up” I saw in the Service Department on the

BSLOL Web Site. There are 2 suggestions which in my opinion are wrong.

(1) Blowing exhaust gases through your engine’s water jacket to remove excess water, may achieve this goal, but it also causes various acids (from the exhaust gasses) to be condensed inside the water jacket. This is not a good idea if you want to protect your engine. (2) Gas tanks should be filled ¾ full. This is very important for the following reason: An empty gas tank is more dangerous than a full one! The reason being is that an empty tank has a much greater volume of gasoline vapor, thus has a greater potential for ignition and explosion. A tank filled ¾ of the way helps prevent condensation of water, reduces the amount of vapor that is contained in the tank, and also leaves room for expansion.
Scott W. Edward

Dear Scott,
The spirit of this article is to have some fun, tease those who contribute to this segment with their questions a little, while trying to remove much of the mystique concerning the maintenance and repairs of our antique and classic motors. I have to confess, my undergraduate studies prepared me with a major in English and grammar. Although our esteemed editor, Peggy Merjanian, may contest my ability to construct a proper sentence without a dangling participle, run-on sentences, or appropriate punctuation, I must suggest your second sentence should read “suggestions that” rather than “suggestions which.” That being said, what you have written is exactly correct; however, with everything in life, there are compromises. With this in mind I offer you the following:

Gas tanks: It is true that gas vapors are much, much more volatile than the gasoline itself. It is also true that today’s gasolines do not hold or maintain their octane rating worth a damn. In addition, gasolines, like most liquids, expand and contract with temperature fluctuations. With warmer temperatures it expands, while the converse is true when it gets colder. Let us not forget that water is in part corrosive to metals, however the key ingredient to rust or corrosion is oxygen (oxidation.) With these facts in mind, here is where my “Lay Up” article has its origins.

In my experience the octane rating loss has created more problems for me than not. I prefer to top off the tank in the spring with fresh gas. If you are fortunate enough to have more than one boat, you may leave your less favorites in the barn for more than one season. I have found it very difficult to properly adjust engines and perform a spring tune-up with something less than the required octane rating. I know that there are gas stabilizers and octane re-boosters on the market, however, I feel these are nothing more than “snake oils” getting the marketers rich.

Compromise number one: Leave the tank empty rather than full. Why? Because the fresh good gas is, in my opinion, better than the old marginal gas. In fact, I have emptied tanks where the old gas from the tank will hardly burn. Gas vapors are and will be present in our tank whether three-quarters full or almost empty. These vapors are almost benign if there is not sufficient oxygen and a source for ignition (the proverbial spark.) Your boat is on a trailer with the battery removed or disconnected, probably on a dirt floor in somebody’s barn. The likelihood of an ignition source is much greater while you are using the boat on the water. Therefore —

Compromise number two: I feel you are at no greater risk with an empty tank. I have never heard of or seen an explosion and/or fire originating from the gas tank — not to say this won’t happen. Gasoline and gas vapors in the bilge create the greater hazard. In my opinion, if there were a greater risk of explosions and fires with empty tanks, the insurance companies would certainly require boat storage operators to top off all the tanks on their customers’ boats.
Condensation occurs when water vapor is cooled
sufficiently to return it back to the liquid state. Where does this vapor come from? The air we breath and gasoline itself. How does it get into our tank? The breather hole located on the side of your boat, the gas pump, and every time you remove your gas cap. You are correct in saying the less gas in the tank, the more potential for condensation to occur. Most of the time however, whatever condensation is present in our tanks will be frozen during the long cold winters in Minnesota.

Compromise number three: I feel the small amount of condensation does not pose a real threat to our tanks and will dissipate into the gasoline in the spring when you top off the tank. It has been my experience that gas tanks go bad on boats that are stored for many many years. The boats that are used on a regular basis seldom have problems. In fact, when a boat comes out of many years of storage for restoration, the damage seems to occur below the level of any remaining old gas, not above. The expansion and contraction of gas in your tank does and will occur to some degree. If you choose to fill your tank in the fall prior to storage even to the top doesn’t bother me a whole lot. It’s been so long since I studied the physical properties of benzenes and their coefficient of expansion and contraction as it relates to temperature, I honestly can’t remember for instance, how much expansion you will have in a typical 20 gallon marine tank. I do know one thing for sure, it is warmer in the fall and in the spring than in the winter, therefore, the volume will decrease in the winter. In the spring it will expand to the volume you had in the fall. If you are contemplating having a three-fourths tank of gas, you might as well top it off. I will admit, on the outside chance you will not use your boat all next year and have the need to store your boat with an abundance of gas, leave a little room at the top. One inch should do it.

Let me address your concern regarding the potential acids being deposited in the water jacket and water passages throughout the block. To this I have to say “boo hoo.” It is true, the combustion process produces a small amount of a type of hydrochloric acid. But never have I seen any affects on the water passages of an older engine. To begin with, these passages are usually so full of mineral and rust deposits, any small amount of acid will never penetrate to the cast iron. Rust is your greatest enemy — especially while operating in salt water. One options is to circulate antifreeze throughout your engine in the fall. Antifreeze has rust inhibitors that help prevent damage to your cooling system. My concern here is, what do you do in the spring? Antifreeze (ethylene glycol) is pretty tough on the environment. In addition, the sweet smell and taste is pretty enticing and deadly to our four-legged friends. If you choose to use the antifreeze method, make sure the thermostat (if you have one) is open, otherwise this stuff will bypass the block and go right out the exhaust onto the ground and not your engine. Remember what I said about most liquids contracting when they get colder. Water expands with tremendous force when frozen. It will expand and break or crack even the toughest cast iron block or water jacket. Either way you choose, make sure you get all the water out, especially the smaller “K” blocks. This is why I say blowing it out gives me that insurance. If you have a large enough compressor, you could use this source as well.

Are you wrong? No. Compromise means there is no exact answer, only what I might suggest as my observations, not the definitive. Remember one man’s opinion, one man’s methods. Thank you for your letter.

Dr. Motorhead

PS: Scott, I would encourage you to become a member of the ACBS & BSLOL There are many benefits and it’s a lot of fun.