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Best defer to manual’s fuel grade

DEAR TOM AND RAY: We bought a brand-new Mini Cooper S. Nobody told us what grade of gas to run it on. Stupidly, we did not look in the manual. We ran it on 87 regular for a little more than a year. When we brought it in for service recently, the dealer told us to run it on 91. He said we most likely have valve carbon buildup. There was no knocking or stalling. I am freaking out that we put a hole in a piston! It is my wife’s car, and she loves it. Please help! – Howard

RAY: Howard, you can stop freaking out. You didn’t put a hole in a piston. If you had, you would have noticed immediately that the car sounds and shakes like one of my brother’s cars.

TOM: Your car’s engine-management system actually adjusts for the grade of gasoline you put in it.

RAY: In the old days, if your car needed 91 and you used 87, you would have caused the engine to “knock” on hard acceleration, on hills or in high heat. That knocking was the sound of pre-ignition, or gasoline igniting when it wasn’t supposed to. That could cause damage to the engine over time. Even in the short run, it could harm things by making your engine run hot.

TOM: But since engines are now managed by computers, yours has what’s called a “knock sensor.” Guess what that does? It senses knock! And if it detects a grade of fuel that’s lower than what’s recommended, it adjusts the car’s ignition timing to make up for it and avoid damage.

RAY: So, why wouldn’t you just use 87 all the time, and save a lot of money on gas? Well, in cars that “recommend” premium fuel, like yours, you can. And I would. Manufacturers say you may notice a small drop in fuel economy and a slight decrease in power, but it’s not enough for most people to notice.

TOM: On the other hand, some cars say they “require” (rather than “recommend”) premium fuel. In those cases, you’re stuck. So our advice is to look in your owner’s manual

RAY: But doing what you did, Howard, caused no damage at all.

TOM: So, for your own mental health, I suggest that you forget this whole incident ever happened. If you have trouble letting it go, visit your Mini dealer’s in-house hypnotist, conveniently stationed next to the service department cashier.

Got a question about cars? Email Click and Clack by visiting the Car Talk website at www.cartalk.com.

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