I have a 2000 gmc sierra 5.3 and been experiencing this problem every winter for last 2 years. I live in so cal so weather here is fairly nice but in the cold winter nights when it gets down to about 40 to 50 degrees at night and my truck has been sitting for more than 4 hours or so it will not turn on. I turn the key and it just cranks and cranks. it takes me, depending on outside temp, 6 to 20 tries to get it to start. I sometimes let off the key and it then sounds like it wants to turn on and I hear the combustion then dies again. I been turning the key let it crank for about 4 seconds the as i left off the key i stomp on the gas and eventually it will turn on. Please help!

Your coolant sensor might be bad. if it thinks it is warm all the time that could make it start hard or not at all when it cold. When the computer thinks it warm it puts less fuel into the engine. When its cold its put more into the engine. I would check the temp that the computer is reading. This can be done with a scan tool that is able to read data.

I have actually already replaced teh temperature coolant sensor and yes problem still exist. I have a flash pak tuner not sure if i could read the temperature with that or not

If you can the temp should be close to the outside temp when it has sat overnight or for a long period of time.

Try holding the throttle open just a tad when attempting to start. If it starts and it seems you have to feather the throttle to keep it going, it would point to a sticking/carb'd up IAC valve.

sometimes they do that when the regulator is bad, or pump is going, try to check the fuel pressure, and see if it goes away fast, it shouldnt. you can smell the vac port for fuel if you smell it, its bad