If your engine is running too lean, it isn’t burning enough gas for the amount of air that is being drawn into it. When your engine runs too lean, it also runs too hot. When your engine runs too hot, it runs less efficiently. Running too lean can also cause the exhaust valves in the engine to lack adequate air flow. A too lean condition in a fuel injected engine is usually caused by either plugged fuel injectors or a faulty mass air flow sensor. This is a condition that is more serious than running rich, and should be corrected as soon as possible.