Mechanically Speaking: Bad gas only exists after long-term storage

" has been very infrequent that I have seen a vehicle with a problem that was related to bad gas."

Twenty-five years of automotive repair and it has been very infrequent that I have seen a vehicle with a problem that was related to bad gas. I can’t even begin to count the number of times customers suggest that their vehicle ran poorly just after filling up their tank. When we get to the underlying problem, it is not likely a problem with the fuel.

Probably the most common bad gas situation is that which occurs after long-term storage. We are talking years without use. For fuel to burn it must have a certain volatility. The volatile components of the fuel allow it to vaporize when it is sucked or sprayed into your engine. Vapors will ignite readily. Liquids do not. Left sitting long enough and exposed to the atmosphere gasoline’s volatile components will disperse leaving a liquid that would be difficult to ignite. It may still work in your vehicle, but your performance will suffer.

Gasoline also degrades by oxidation. This causes much bigger problems in your gasoline engines; be it your cars, trucks, boats, lawn mowers, quads, sea-doos, and skidoos. The presence of oxygen with your fuel in the long term (roughly a year) will cause the fuel to form gum and varnish. These substances plug passageways in the fuel system. Varnish will plug carburetor jets (metered fuel passageways), injector nozzles, and fuel filters to name a few items. Removing this stuff will likely require a physical process or replacement of parts. They are not easy to dissolve with any fancy chemicals.

The most common gasoline problem that occurs after refilling your fuel tank is getting fuel that is mixed with water. Storage facilities like gas stations can get water in their tanks, and that water can get in your tank. It more likely will end up there due to problems with your fuel system, though. Water does not dissolve in gasoline, and it is heavier than gasoline; therefore it hangs out at the bottom of the fuel tank. The bottom of your fuel tank is where the fuel pump picks up the fuel. When the amount of water is big enough, and the temperature drops below freezing that puddle of water will freeze, and the pump will be sucking on an ice cube.

Water dispersed throughout the fuel system also causes corrosion in any steel parts. Once any internal components of a fuel system are rusting life with your vehicle will be less than pleasant. Your vehicle will require a steady diet of fuel filters to keep it running half-decently until you actually remove the internally rusted parts.

As most of our gasoline now contains ethanol (ethanol has an affinity for water) the water problem is more prevalent.

So what can you do to cross bad fuel off your list as the reason your vehicle or gasoline powered implement does not run well? Keep the fuel fresh. When storing gasoline or any implement with a gasoline tank keep the tank full and use a fuel stabilizer product to reduce the rate of oxidation and absorb the water.

When your daily driver does start to exhibit problems in the way it runs it is not likely bad gas but more likely a developing issue that is not going to be cured with the next tank of gas.

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