Ensuring stoichiometry is kind of a big deal. When the engine is cold, the mixture needs to be rich or it won't start. Engines with carburetors use a choke for

Ensuring stoichiometry is kind of a big deal. When the engine is cold, the mixture needs to be rich or it won't start. Engines with carburetors use a choke for that - a valve that blocks some airflow so there's more vacuum, causing the engine to suck in extra fuel. Electronic fuel injection can be much more precise about this, minimizing the amount and duration of enrichment. Changes in air pressure and temperature change the ideal air/fuel ratio. Driving over a mountain will make an engine without the ability to react to run at a suboptimal air/fuel ratio. Catalytic converters require that the ratio be kept very close to optimal to operate effectively. A small deviation for an extended period of time will result in a large increase in emissions and may even permanently damage the catalyst.