Questions - Should I drain the gas from my truck or fill the tank full before winter storage? Should I start the engine weekly or monthly during the winter even if I don‘t plan to actually use the truck?
My advice - Drain the tank and let the truck hibernate for the duration. Add gasoline stabilizer if the tank is not completely empty. Take steps to prevent an accidental fire.

We have always been told to store a vehicle with a full gas tank to prevent condensation inside the tank. That's terrible advice. I advise storing with an empty tank to prevent a fire - the worst scenario. We read reports every year of garages with expensive collections being destroyed by fire. If you remove this flammable liquid, you lessen the chance of fire devastating buildings and vehicles.

Gasoline is different now. When lead was removed, other nasty ingredients were added. It now has octane boosters that are rough on rubber parts by making them soft and spongy and increasing the probability of them failing and dripping fuel on the garage floor.

Oxygenates, such as ether or alcohol, which are probably included in your fuel, put oxygen right inside your tank to corrode it when it is full. On the positive side, the oxygen additives act like dry gas, keeping larger amounts of water in solution than older-formula fuel.

Gasoline is like milk; they both spoil quickly. Some recommend storing gas no longer than three months, others say six months. These recommendations might actually be in agreement if one considers that three months may elapse between refining and distributing. One report concluded that modern, reformulated gas has a shorter shelf life than “classic” gas, but another source disputed this claim. The manual for my new chain saw says to drain the fuel tank and run the engine until the carburetor is dry if storing for three months or longer.

Fuel stabilizers designed to prolong the shelf life of gasoline are helpful. I recently asked a chemist at STA-BIL to describe how the fuel stabilizer worked. “Hey, dude, can you prove the stuff in this bottle ain't snake oil?” I said.

He convinced me it really does slow down gasoline deterioration. There is a standard test for this called the ASTM D525 “Oxidation Stability Test.” A gasoline sample is put into a container with pure oxygen under 20 atmospheres of pressure. When the sample starts to polymerize (oxidize), the temperature goes up. This is when gum starts forming. Plain gas takes 100 to 200 minutes to start polymerizing, but gas with STA-BIL lasts 1,200 to 1,500 minutes. Sounds good to me. My chain saw two-cycle oil contains stabilizers to prolong the life of the mix.

Gasoline also comes in winter and summer formulas, with the difference being its vapor pressure. The refineries switch the blends in increments in spring and fall, according to anticipated temperatures. (Seasonal blends vaporization characteristics chart)

A summer blend will make vehicles hard to start in cold weather because it evaporates too slowly. Slower-evaporating fuel lowers evaporative emissions and is mandated by the government. But we need gas to vaporize to start a cold engine. If it evaporates too slowly during start-up, the engine will flood and not start. That washes oil off the cylinder walls and dilutes the motor oil with raw fuel. Winter-blend gasoline evaporates faster for easy cold starts, but if your tank is full of this blend at the Memorial Day parade, you are ready for a vapor lock. It is necessary to have a fresh tank of summerblend gas for trouble-free summer driving. If you work your truck during the winter, it's necessary to have winter-blend fuel in the tank or starting is extra hard. (Gasoline Additive Chart)

Fuel continuously evaporates out of the tank. This is good and bad. It does keep a slight positive pressure in the tank, which helps keep moisture out. That's the good news. However, only the easily evaporated components evaporate. This is the same fraction that will catch a spark to fire a cold engine. We are left with the heavier components that work fine in a hot engine, but won't start a cold one.

Chevrolet fuel tanks prior to the late 1950s came with a drain plug at the bottom. I add a petcock so I can easily remove old gas. Old trucks stand high enough to make it easy to climb under them to remove fuel without jacking them up.

One criticism of draining fuel is that the carburetor accelerator pump dries out. This can be remedied by adding a squirt or two of motor oil to the float-bowl vent before the engine expires for the last time before storage. This has the added advantage of misting oil over the valves and combustion chamber for extra corrosion protection during storage. If this is overdone, expect to clean plugs in the spring and clouds of smoke on start-up.

Check the fuel cap. If moisture is going to enter the tank, it has to get past the cap. Replacement caps for Ford, GM, and Dodge are inexpensive and readily available going back to the 1930s. Stant G25 covers Ford 1937-50 and Stant G23 covers Ford 195 1-70, Dodge 1928-60, and Chevy 1937-70. Even parts trucks stored out back should have good gas caps.

Also, check the fuel filter. If it is between the tank and fuel pump, move it up between the pump and carburetor. A mechanical fuel pump is much better at pushing fuel than pulling it. Keep the suction side unrestricted. What about starting the engine during winter even if you don't plan to go anywhere? The rule of thumb is to run the truck hard enough to thoroughly warm it up. This evaporates moisture of combustion from the crankcase and exhaust system. If your truck has leaky piston rings, they will leak some raw gas into the crankcase. This too should be evaporated away by running the vehicle under load before shutting it down.

Sniffing the oil dipstick is a crude but effective guide. A Mobile chemist told me a human nose can pick up parts-per-million gasoline concentrations. I believe a short winter run in the driveway does more harm than good. Wait on the start-up until you can run ten or 20 miles on the highway. Your truck doesn't start to burn off moisture until it is really warm.

(Gasoline Specifications and Their Importance Chart)
(Environmentally Driven Gasoline Changes and Their Results Chart)
(Effects of Gasoline Volatility on Vehicle Performance Chart)
(Fall Maintenance Suggestions List)

Summary: If you don't start and run your truck during the winter, run the fuel tank dry or almost dry and add some fuel stabilizer. Fuel leaks are a major fire hazard—sniff them out. Plan gasoline purchases so the fuel will be used up within three months.

Bob Adler is owner of Adler's Antique
Autos, Stephentown, New York, and
specializes in GM truck restoration.
He can be reached at 518-733-5749.
Email