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2004 Forester XTi
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I have been running on 93 octane (ProECM chip installed on my car) and I pump at Holiday gas stations. They use specialized gasoline that contains less sulfur additives (~20% less) which allows a cleaner fuel burn that causes less emissions. They are so-called "Blue Planet" gasoline, and the price is actually cheaper than the conventional gas stations.

Back to the story, my fuel was really low. I didn't think I could have made it to a Holiday station so I pulled into a Super America gas station. In recent history, S.A. was tested to have the best quality fuel in this part of the country but only has a max of 92 octane. I poured in some fuel cleaner, filled up all the way, and drove to class.

Three cold hours later, I drive home slowly accelerating and stop at the first light. I go on the green light and when I shifted to second @ 2500 rpm, the rpm tach dropped really fast. I didn't notice, so I shifted and pressed the gas. The car thumped as the rpm needle jumped from 1000 rpm to 1800 rpm. It happed again on my shift to third. Later when the car was fully warmed up, I shifted to second and the rpm needle stayed up at 3000 rpm... causing the car to jump forward. Today, it hasn't happened again.

It left me wondering if it was the gasoline that caused this; or could it have been a blocked PCV valve (stock)/bad fuel filter (12000 miles ago)/fuel additive/or something? Could it be that my ECU is learning the fuel curve of the lower octane rating? Different fuel added? Has this happened to others?

~My car is at 52500 miles, and I used an Amsoil fuel cleaner that you pour in 1 oz. for every 5 gallons of gasoline.
 

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Highly unlikely the gasoline had anything to do with it, unless there was water in the tank or something.

91 octane is pretty much useless for a non-turbo car, as octane only prevents detonation created by higher intake temps that are often associated with very high compression engines or turbo engines.
 
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