I have a chevy truck and when it is 40 degrees and lower it starts up and idles rough until it has warmed up. do you know what might be the problem here?

Well I would need to know more about the vehicle

And how does it idle after its warm? Is it rough?

Is the check engine light on?

How cold are you talking? 40 degrees or -10 degrees

Leaking intake manifold (vacuum )

Try to get it running a bit to warm up the exhaust. Take a point and shoot thermometer and take the temp of the cat. If temps exceed 400 degrees I would start to suspect a plugged cat.

it is the cam position sensor...the telltale sign of the failure is that it happens when the vehicle is warm and has been driven for a while. Sensor is about 40 bucks at the auto parts store, make sure the replacement sensor is metal, not plastic.
The main symptoms of this sensor failing are rough starts, rough idling and not starting at all especially when warm.

Check the water coolant thermostat. If it is stuck in the "open" position you are heating up all the water in the coolant system and this will take time. The thermostat keeps the water in the engine until warmed up to almost normal running temperature, only then does it allow the coolant to enter the radiator.

Could be the fast idle sensor check to see that the vacume line is hooked up properly.

that means could be vacuum leak

Make sure all the glow plugs are working.

HAVE THE COOLANT TEMP SENSOR CHECKED TO SEE IF IT IS FUNCTIONING PROPERLY.HOPE THIS HELPS. BEST OF LUCK. LET ME KNOW?

Mine did the same thing at 16K miles, I cleaned the throttle body and the ploblem went away. Took me about 30 minutes.