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In simple terms, an engine needs a perfect mix of air and fuel for optimum combustion and longevity. Running “lean” (more air than the ideal “stoichiometric” 14.7:1 air:fuel ratio) will actually increase power, but at the cost of reliability and the risk of catastrophic engine failure. In real-world driving conditions, the AFR is typically kept somewhere between 12:1 and 13:1 (slightly rich) in order to keep exhaust temperatures lower and to compensate for fuel quality variances. The stoichiometric AFR for diesel vehicles is typically 14.5:1.

"All of a sudden I’ve got a lot more power, so I’d better check the air-fuel ratio to make sure I’m not about to blow this thing up!"