[Christ]

Originally Posted by Ford Man

Warm air is less dense and has less oxygen molecules therefore less oxygen in the exhaust stream for the 02 sensor to detect and the IAT sensor sends a warmer temperature reading to the ECU, which fools the ECU into thinking the outdoor temperature is hotter than it actually is and leans out the mixture. All cars run richer when cold air is being introduced into the engine. Many have found that warmer IAT's increase their mileage.

They don't run richer, they just add more fuel to compensate for the presence of more available air. "Richer" suggests an AFR different than desirable, as in "too much fuel for available air mass".