Springtime engine troubles? It could be stale gasoline | Fuel Facts

Let’s say that it’s springtime, and your car–or even your lawn mower, rototiller or whatever–won’t start. You’re greeted with an engine that simply will not turn over. 

All seems good with the ignition system, and there isn’t a problem with your airflow. Initial signs point toward an issue with the fuel. Now what?

“If it’s just been one winter,” notes Zachary J. Santner, senior specialist of quality at Sunoco, “the gas probably hasn’t varnished up.” 

Instead, he continues, the fuel has likely started to go stale. The technical reason behind fuel going stale: The light ends, known as short-chain aliphatic hydrocarbons, have evaporated first, leaving behind the long-chain aliphatic and aromatic hydrocarbons that are harder to ignite.

The easy fix, he notes: Top off with fresh fuel and then, assuming the engine comes back to life, run through that entire tank. Don’t give that older fuel more time to degrade, he recommends. 

Could the issue instead revolve around the fuel absorbing too much moisture while it sat in the tank? That’s also possible, he explains, although he reminds us that fuel containing a little moisture–say, up to about 2%–can still ignite. In this case, the engine will simply turn that moisture to steam and send it through the exhaust.

If the fuel contains more than about 2% moisture, he continues, then phase separation can occur, with the water settling to the bottom of the tank–and also, he notes, the fuel bowls. 

If there’s a lot of water content, he says, then you need to drain the tank. If the water content isn’t so bad, an additive containing methanol can be used to reblend the two liquids. (Thank the property of miscibility for this reblending, although, Santner continues, note that methanol serves as a degreaser.)

A long-term fix to avoid fuel issues? Keeping a fresh supply of fuel in the tank, he says.

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