How long does the engine need to idle before driving in cold weather?

Hello, There is no real reason to let the engine idle before running, unless you are running forged pistons. Forged pistons, which are not installed on many factory vehicles, expand much more than cast pistons, and they let you know it is ok to drive when they stop making a rattle/slap noise. As long as your jeep is factory stock, i.e. no modifications, you are fine to drive the vehicle once the engine "idles down". This refers to the engine idle speed at startup. When you start the engine, the engine speed will remain higher than normal, then slowly reduce until it is somewhere between 600-850RPM. If you do not have a tachometer, just listen for the engine to get quieter, and off you go.