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What does it mean when a vehicle runs lean?

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What does it mean when a vehicle runs lean?
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The air fuel ratio has too much air and not enough fuel. Too rich would be the opposite.
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It means it uses a close to perfect amount of gas to produce the effects it is supposed to. A car that is not running lean will use more gas to do the same things, and probably produce more toxins per mile.
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Or, if the air source is normal, running lean (air-fuel mixture) means the fuel part of the air-fuel mixture is below normal. . . Is the vehicle driving lean because it is lean and mean?
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gniker is correct. It means that your air/fuel ratio has more air than fuel.
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