Ethanol In Pump Gasoline Is Why Small Engines Sit Badly

A mower or blower that ran fine in October and refuses to start in April has almost always been damaged by the fuel left in it. The mechanism is chemical and it is predictable.

Ethanol pulls water out of the air

Most gasoline sold in the United States contains ethanol, which is hygroscopic: it absorbs moisture from the air in the tank above the fuel.

A vented tank breathes as temperatures swing, drawing in humid air each cycle. Over a winter in an unheated space that adds up to a meaningful amount of absorbed water.

Past a certain water content the mixture separates, and a layer of water and ethanol settles at the bottom of the tank. That layer is what the fuel pickup sits in.

Gasoline oxidizes into varnish

Separately from the water problem, gasoline is a mixture of volatile compounds that evaporate and oxidize over months. The lighter fractions that make an engine start easily go first.

What remains is heavier and gummier, and it deposits as a sticky varnish inside the fuel system. Carburetor passages are the smallest openings in that system and clog first.

A carburetor jet blocked by varnish produces exactly the spring symptom owners describe: the engine turns over normally, fires briefly, and will not keep running on its own.

Rubber and metal are attacked differently

Older fuel lines, primer bulbs and diaphragms were formulated before ethanol blends were standard, and they harden and crack when exposed to them over time.

The separated water layer, being denser and sitting at the bottom, corrodes steel tanks and fittings from the lowest point. Rust particles then travel forward into the same small passages.

Replacement parts for equipment still in production are generally made from ethanol-tolerant materials. Older equipment is where the material incompatibility shows up first.

Storing empty and storing full both have logic

An empty tank has no fuel to degrade, but it does have humid air and bare metal, so condensation and rust become the exposure instead.

A full tank leaves little air space and less room for moisture, which suits a sealed metal tank. It only works if the fuel itself is treated so that it does not degrade in place.

The third option is running the system dry after treatment, so that no fuel remains in the carburetor bowl where varnish does the most damage. Manufacturers differ on which they recommend.

Stabilizers work before degradation, not after

Fuel stabilizers are antioxidants that slow the reactions producing varnish. They are added to fresh fuel and circulated so the treated mixture reaches the whole system.

Added to fuel that has already gone stale, they do nothing useful. The compounds they prevent have already formed and are already deposited.

That timing is the entire practical point. Stabilizer is a task for the last mowing of the season rather than a remedy for the first attempt in spring.