Storing cars with aviation gas

Question:

I recently visited a significant car and truck collection in Newberg, Oregon, and got a chance to talk to the owner and also the mechanic who restores and maintains the fleet. Since some of the vehicles were obviously not driven very often (annual parades or less for some of them) I asked what they did about fuel getting old in their systems. They told me that they don’t use normal pump fuel. They use nothing but aviation fuel because it does not leave any residue when it dries out. If a fuel system dries out, they just add fuel when they want to operate the vehicle and seldom have any problems. The price of avgas is nearly $5 per gallon, but it seems like it could be used for just the last fueling of the season for vehicles that are parked for extended periods and it could save a lot of swearing and frustration on the next use of the vehicle. It could also be good for the last fueling of small engines that hibernate over the winter or longer. It is probably cheaper than fuel stabilizer and maybe better if the non-use period ends up to be years.

Although I have not used it yet, the fueling company at a nearby small airport was happy to sell me fuel in cans “for my float plane.” It would probably be illegal for them to put it directly in a vehicle because of road tax issues.

Have you ever heard of this and do you anticipate any problems with it? High octane doesn’t hurt anything. Any thoughts on this subject for fuel-injected systems that might dry out? Also, contrary to convention, it seems like an empty fuel tank might attract less condensed moisture than a full one because of less thermal mass. An empty tank would be ambient temperature quickly and just breathe, while a full tank would be cold longer as ambient rises and could be more likely to condense moisture from the air. Your thoughts or experience on this theory?

Answer:

The single best thing you can do for a car that’s in storage is start and run the engine on a regular schedule, preferably once every week or two.

If that’s not feasible then you should take steps to protect the car from decay. You will not damage a vintage automobile engine as a result of using aviation fuel. A higher-than-needed octane rating will not hurt an engine. Aviation fuel will, however, hurt newer cars designed to run on lead-free fuel because aviation gas does contain lead. Lead will cause the oxygen sensors and catalytic converter to fail. At the same time, it is, in fact, the lead, more than anything else, that makes aviation fuel a suitable choice for vintage cars that will remain idle for extended periods of time. The lead actually serves as a preservative for the gasoline. You can, however, achieve the same thing with leaded racing fuel. I had leaded racing fuel that was nearly six years old and though its color changed it smelled good and worked fine in my cars.

I spoke with a chemical engineer at Sunoco who explained that the dye used in the fuel for identification purposes would eventually fade, thus explaining the color change, but the fuel itself remains perfectly good for a long time.

Regarding the empty vs. full fuel tank, an empty fuel tank is less susceptible to condensation formation and resultant corrosion compared with a partially filled tank. By the same token, however, a completely full tank is essentially immune from condensation forming on the inside because the fuel displaces all of the air in the tank. So, at least as far as condensation and corrosion are concerned, you’re better off with a tank that’s either completely full or completely empty. When a fuel system is completely empty for an extended period of time, however, you are likely to end up with leaks when fuel is introduced back into the system. This is because various seals depend on the fuel to retain their size and shape.

Once they dry out, shrink and distort, they probably won’t go back to their original size and shape.