Like the Weather, Gas Composition Changes With the Seasons

Question: My 1992 Chevy Lumina has started running roughly and my gas mileage has grown worse. The dealership can’t find anything wrong. I am wondering whether it has anything to do with winter fuel. What exactly is winter fuel and what good is it, anyway? P.A.

Answer: The federal Environmental Protection Agency requires gasoline companies in 39 major cities, including Los Angeles, to modify their fuel chemistry in the winter months to reduce emissions of carbon monoxide.

The EPA is concerned about the winter months because cold engines produce more of the gas and because typical winter weather causes temperature inversions close to the ground in the late afternoons.

With many vehicles on the road during evening rush hours, there is concern that inversions could trap carbon monoxide and increase its concentration to levels that could impair health. EPA studies appear to show that use of winter gas can lower carbon monoxide concentrations in urban air by 8% to 10%, enough to help susceptible people, according to Jack Segal, an Atlantic Richfield gasoline expert.

“These changes in formulation are a very painless way to reduce pollution,” Segal said. “You don’t have to do anything to your car or buy fancy new vehicles. Basically, it is a very consumer-friendly way to reduce automotive pollution. It happens the minute you fill your vehicle.”

Federal standards limit carbon monoxide emissions in new cars to 3.4 grams per mile, though the typical new car puts out half that amount. But old cars can spew 10 times that amount.

Fuel is modified for winter by adding methyl tertiary butyl ether or MTBE (a member of the alcohol family) to the fuel. Sometimes, ethanol is used.

Under federal rules, refiners are supposed to add MTBE to a level of 15% of the fuel by volume, though California officials want to reduce that because MTBE causes increased output of oxides of nitrogen, another serious pollution concern.

MTBE or ethanol should not directly cause a rough idling or running condition in a 1992 model year car, though both can affect older cars. The products can raise the fuel vapor pressure, a measure of its boiling point, and on an unseasonably hot day could cause vapor lock in the fuel system.

The fuel economy loss you note, however, is the fault of the winter fuel. Ethanol and MTBE are lower energy fuels than regular gasoline. Typically, the winter mixture reduces fuel economy by about 3%, Segal said. At the same time, the ingredients in winter fuel boosts its price at the pump by 2 to 5 cents, depending on supply and demand.

If your rough running condition persists, you might consider using a fuel injection cleaner or change gasoline brands. If the fuel injection system or ignition system is malfunctioning, the car’s diagnostic system should give mechanics an indication of the problem.