Q: Why does my 2012 nissan altima Rev while engine is cold? It just started doing this with cold weather.

Modern vehicles have what we sometimes refer to as a "warm up program". The transmission will hold engine revs a bit higher until the engine warms up. This is to warm up the engine and catalytic converters faster for emissions reasons. When the engine is cold, it needs more fuel to run, causing more emissions out of the tail pipe. Warming it up faster, reduces the length of time this happens. However, if the revs are surging up and down on its own, other factors may be at play. For example, there may be a vacuum leak in the intake system allowing more air into the engine. This can cause a lean condition which is not ideal when the engine is cold. Running lean can cause sensors like the throttle position or idle air, to "hunt" for the correct idle speed and give you a surging idle.

Other components like the coolant temperature or ambient air/barometric pressure sensors can be at fault. If these or any other sensor are registering improper values during cold running, the engine control unit may hunt and surge for the proper idle speed, whether you are at a standstill or coasting with your foot off the gas pedal. A certified technician can scan the system for faults as well as proper sensor values and input to determine why the idle is surging.