The right fuel in the Fall might help your engine start in the Spring | Fuel Facts

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Car won’t start after a few months in storage? It could be the gas. “But,” you counter, “there’s fuel in the tank and it’s getting to the engine.” But is that fuel still good? 

The Fuel Could Be Stale

Fuel goes stale as it sits due to a physical change. Gasoline is formed of aliphatic and aromatic hydrocarbons–think of them as open chains and closed rings. Those light ends, technically known as short-chain aliphatic hydrocarbons, evaporate first, leaving behind the long-chain aliphatic and aromatic hydrocarbons. These are harder to ignite because they need a higher temperature to vaporize enough fuel to support combustion, explains Zachary J. Santner, technical specialist at Sunoco Race Fuels.

  • The Fix: If the tank isn’t completely full of stale fuel, you might be able to simply add enough fresh fuel to rejuvenate things. Purging the fuel lines to the engine might also be necessary. 
  • The Prevention: Pump gasoline contains antioxidants that are consumed as they react with oxygen. These antioxidant packages aren’t inexpensive to produce, Santner tells us, so you’ll find higher doses in race fuels and high-octane pump gasoline. 

The Fuel Could Be Wet

Most pump fuel contains ethanol, and that ethanol attracts the moisture found in the air that we breathe. Water doesn’t burn nearly as well as gasoline, of course, and causes other issues in the fuel system. 

  • The Fix: “It could be as simple as draining the carb and getting fresh fuel in there,” Santner notes. Or you might need to drain the tank. 
  • The Prevention: Fuel that’s low in or free from ethanol will attract less moisture. Sunoco Race Fuel’s Optima, for example, is specifically designed for storage: It’s ethanol-free and rich in antioxidants. 

The Fuel Could Contain Varnish

As gasoline evaporates, it leaves behind gum, a technical term for a varnish residue that can clog small ports in the fuel system.

  • The Fix: If adding fresh fuel doesn’t fix things, you might end up replacing fuel filters and cleaning injectors or carburetors. 
  • The Prevention: “There’s a spec for how much dissolved stuff can be in fuels,” Santner says of substances like gum. The standard test for pump gasoline allows 5 mg of gum per 100 mL of gasoline. Sunoco’s race fuels contain a tenth of that, he notes. 
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