[HyperMileQC]

Lean burn is a special mode the engine goes when there is low load on the engine. The air to fuel ratio goes from 14.7:1 to 20:1 and more sometimes. You burn lean so you take less gas.

But because of the higher emission of Nox due to higher internal combustion temperature, the insight is equipped with a special catalysor to purge the Nox. This is why when you are in lean burn sometimes the mpg drop and the engine suddendly gained more power for 10-30 seconds and then it return to lean burn operation and the mpg increase but you loose power.