1997 gmc sierra pu.5.7L won't start in cooler weather when left outside or sits idle for a couple of days. The fuel pressure reg. has been changed with no apparent change. If I warm-up the fuel lines near the entry into the manigold it will start.This is not an ignition problem.Any ideas?

You need to have the engine coolant switch checked,since the computer relies solely on sensors,the ect tells the computer how cold or hot it is so the computer will make it a rich or lean mixture to start the engine,if it tells the computer that it is 75 degrees the computer will lean the mixture like starting a car in the winter without a choke.

Most fuel injected cars have a 5th injector that is just for starting. It often is powered off the starter. It usually also has some sort of thermal time out switch, so you don't flood the engine. So you need to pull out this cold start injector to watch if it sprays or not when engine is cranked. If not, then decide if injector or shut off timer by shorting the timer. Sorry, but not GMC specific answer.

Hell no h2o gas line antifreez

We had this same problem with our 97 Sierra Z71 5.7 Vortec. Last Winter if truck was outside for more than a couple hours it would not start. We towed it home and put it in the garage. Went out next morning and truck started. Left truck outside and it would not start. It started up fine in the garage or on warmer days. We replaced the electric fuel pump in the tank, and new fuel filter. I also made sure the ground wire to it was making good connection to the frame. (Someone before us did that) Afterwards the truck ran great no matter how cold it got outside.