Rough idle and no power when engine cold

[Asfalto]

Hi everyone!
Here is the new issue on my 1998 tahoe 4dr 5,7 4x4:
when engine iscompletly cooled down, cranking takes more than normal and, after that, engine runs very rough (5 or 6 cylinders to me) with no power; sometimes it hardly move the truck, sometimes it stalls.
when hot it starts immediately and runs fine and smooth.

note:10 years ago i've installed an alternative fuel system (LPG: liquefied petroleum gas) on the truck, it is normal here in italy since the gas price is 7,16$ per gallon.
This gives me the opportunity to check the engine in the LPG mode and it runs fine at any engine temperature.
example: start on gasoline-->rough idle-->switch to LPG-->fine and smooth
This leads me to think that it's not an electrical or ecm problem but a fuel related problem.
What can cause this issue only when the engine is cooled down?
thank you

 

[bigfootchiro]

Are the issues you are speaking of occurring using gasoline or LPG?

 

[Asfalto]

Only on gasoline and when engine is cold (outside temperature 60 or more).
If i switch to LPG runs fine, if i switch back to gasoline it runs rough again.
This until engine warms up; then runs fine either gasoline or LPG.
thanks

 

[HiHoeSilver]

Only on gasoline and when engine is cold (outside temperature 60 or more).
If i switch to LPG runs fine, if i switch back to gasoline it runs rough again.
This until engine warms up; then runs fine either gasoline or LPG.
thanks

Check your pressure at the rail. I'm guessing fuel pump.

 

[Asfalto]

i'll check as soon as possible and let you know
thank you