This is from Chevron:
Oxidation
Except for any added oxygenates, gasoline is made up almost entirely of hydrocarbons–molecules constructed from the building blocks of elemental carbon and hydrogen. Hydrocarbons, as a class, are chemically stable molecules.
However, there are types of hydrocarbons (olefins and diolefins) that can combine slowly with the oxygen in the air ("oxidize") at ambient temperatures. The products of the reaction are larger molecules, collectively called "gum."
The gum-forming reactions become faster as the temperature of the gasoline increases. This is why this bulletin recommends controlling the temperature of stored gasoline.
Most gasolines contain negligible amounts of gum when they are manufactured, and most contain special chemicals ("stabilizers") to retard gum formation. It is the stabilizers that make it possible to store Chevron gasolines for a year when the conditions are good.
Soluble Gum
The gum formed by oxidation is usually soluble in gasoline. However, it remains behind as a sticky residue when the gasoline evaporates. Since gasoline begins to evaporate in the carburetor of a carbureted engine or in the injector of a fuel-injected engine, a gasoline containing soluble gum may leave a deposit on these parts and on the intake valves. These deposits will be in addition to the deposits normally formed by a gum-free gasoline — a formation triggered by the elevated engine temperatures.
Modern engines are designed to run best when vital engine parts are clean. Carburetor and fuel injector deposits can cause hesitation and stumbling on acceleration, lower fuel economy, lower power output, and higher emissions of hydrocarbons and carbon monoxide. Excessive intake valve deposits can cause many of the same performance problems, plus higher emissions of hydrocarbons, carbon monoxide, and nitrogen oxides.
Because the Federal Environmental Protection Agency recognizes that fuel system deposits increase emissions, they require all gasolines to contain a deposit-control additive. All deposit-control additives keep deposits from forming; the best ones clean up deposits formed by lower-quality gasolines.
If the gasoline contains a lot of soluble gum, the normal level of deposit-control additive may not be sufficient. This is why Chevron recommends treating a tank of gasoline with an extra dose of deposit-control additive if a vehicle displays driveability problems after being stored. (Note: Chevron does not recommend adding a deposit-control additive to the fuel for a two-stroke-cycle engine.) The gum-forming reactions become faster as the temperature of the gasoline increases.
Insoluble Gum
Severe oxidation of gasoline may produce insoluble, as well as soluble, gum. The insoluble gum will take the form of brown or black particles which float in the gasoline or settle to the bottom of the container.
When an engine is fueled with gasoline containing insoluble gum, the fuel filter will remove the gum. If the engine has an in-tank fuel pump, the screen on the pump's feed also may capture some of the gum. However, these devices can become plugged if the gasoline contains too much insoluble gum. This will cause the engine to lose power or stall because it is starved for fuel. Adding a deposit-control additive will not keep insoluble gum from plugging filters and screens.
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