1999 silverado is hard to start when engine is warm, and idles rough for a couple of seconds after starting

You may have a leaky fuel pressure regulator located on the LH side of the engine on the side of the fuel rail. The internal diaphram can leak fuel which then gets into the vacuum line attached to the regulator. From there the leaking fuel travels through the line and gets into the intake manifold causing the motor to be slightly flooded when you restart it. The regulator is round, about 1-1/2" diameter, and about that long and has a small 1/8" vacuum line attached to it. After the engine has been off for a few minutes if you remove the vacuum line from the regulator and find even a trace of fuel in the line then you've found your problem. Typical symptoms for a leaky regulator are: Starts fine cold and will restart fine warm if the engine has only been off for a minute or so. Restarts hard warm if the vehicle has sat for several minutes or up to an hour. May produce a bit of black smoke during the warm/hard start.