Gasoline volatility
by Jim Kerr

Many auto enthusiasts greet the warm weather of summer by taking their prized vehicles out of storage and cruising to car shows and show and shines. These cars usually have not been operated for many months, and they often run rough and perform poorly when first started. Anybody who has to start any gasoline powered device after it has been stored can have similar problems. This includes lawn mowers, roto-tillers, snowblowers, generators, as well as cars and trucks. It may all be caused by the fuel!

Gasoline is a very complex substance made up mainly of carbon and hydrogen atoms. Not all the atoms are combined the same way. Some parts of the gasoline will evaporate very easily. We say these parts have a high volatility. Other parts of the gasoline evaporate much slower. When gasoline is stored in an unsealed container such as a carburetor or gas tanks on most collector cars, only some of the gasoline evaporates. The high volatility parts evaporate first. Then the parts with medium volatility, and then over a period of time that may be several years, the parts with low volatility evaporate. Only gummy sediment will be left behind.

For a gasoline engine to start and run well, the volatility of the gasoline must match the operating conditions of the engine. For example, if all the gasoline was of low volatility, then it would not evaporate easily at low temperatures and we would have trouble starting the engine when it is cold outside.

On the other hand, if the gasoline was all made up of high volatility compounds, then the gasoline would evaporate too easily during hot weather and vapour lock could occur. Vapour lock refers to the situation when the fuel boils in the fuel lines so it cannot be delivered correctly to the engine. When vapour lock occurs, the engine operates as if it was running out of fuel.

The gasoline we buy at the pumps is not the same year around. During the summer, the refiners supply gasoline with a lower volatility to reduce vapour lock problems on our vehicles. This low volatility gasoline also helps protect our environment by reducing hydrocarbon emissions caused by gasoline evaporating. During the winter months, high volatility gasoline is produced to help our cars start better during cold weather. If we try to use gasoline made for warm weather use during a winter cold snap, then the engine may not start! During fall and spring, the gasoline is blended to an intermediate volatility to enhance vehicle driveability during those climatic conditions.

Although the refiners may change the volatility several times during the year, typically they supply high volatility winter fuel from November to the end of March. During April, an intermediate blend is supplied, and lower volatility fuel is supplied from May to September. An intermediate fuel is again supplied for October as the weather turns cooler. Most vehicles will operate well with the different fuels, however unseasonably warm or cold temperatures can cause an engine to hesitate or stall. This most often happens during the spring and fall. The refiners cannot react fast enough to supply the fuel required for only a day or two of unseasonable temperatures.

Putting an engine back into service after storage will mean that in most cases the fuel is not the correct volatility. To enhance your engine's performance and reduce driveability problems, use only fuel produced within the season you are using it. This will ensure you have the correct fuel volatility. With today's high fuel prices, there may be the urge to buy and store fuel when you find a deal. Don't bother. Sometimes the fuel is lower priced just because the refiners are trying to dispose of fuel that is the wrong volatility. Have you ever noticed gas price wars tend to occur during the spring and fall?

If you have a tank full of old fuel and your engine is running badly, a tank of fresh fuel can make a dramatic difference. Draining the old fuel out and mixing it with fresh fuel in a vehicle that is driven regularly is one method of disposing of it, but it can be very dangerous! A spark, flame, hot surface, or even static electricity can ignite gasoline fumes causing an explosion or fire. Gasoline has more explosive power than dynamite but we sometimes take it for granted because it is available everywhere. I know of two commercial repair shops that burned to the ground because gasoline came into contact with an ordinary trouble light!

Many small engines have drain plugs for the fuel tank and carburetor so they can be easily drained before storage. Unfortunately, automobiles do not have this feature. Store your vehicles with the fuel tanks almost empty and then fill them up when you are ready to drive. If the tanks are full, keep adding fresh gasoline as the tank empties to help your engine's performance.

On any engine that is started after being stored, watch for fuel leaks at all hoses, connections, and gaskets. The hoses and gaskets may shrink because they are exposed to air instead of fuel. Then a leak may develop. These types of leaks usually show up during the first tank full of fuel, so pay special attention then.

One more word on safety: Keep a fire extinguisher handy when working on any gasoline fueled engine. You probably won't need it, but it is too late to buy one after a fire starts! Carrying one securely fastened inside any vehicle is a little bit of extra insurance.

Jim Kerr is a master automotive mechanic and teaches automotive technology. He has been writing automotive articles for fifteen years for newspapers and magazines in Canada and the United States, and is a member of the Automotive Journalist's Association of Canada (AJAC).

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