Cold startup ?

[Fran]

Why is a startup with the car cold (not recently operating) louder than one that is after the car has been running for awhile? Is it because:

Air fuel mixtures different for a cold startup?
Different clearances in a cold exhaust system vs one that has been operating?

Always wondered about this.

 

[PhillyCobra]

When cold, the computer commands a higher idle speed and a richer mixture than when the car is warm. This keeps engine running smoothly until it warms up and rapidly heats the cats to cut emissions.