My 99 chevy silverado 1500 with 5.3 idles rough only when its cold until it warms up to temperture it only does the first start of the day if the teperture outside is 50 or below

Leaking intake manifold (vacuum )

My last silverado was a 99 and i changed out my fuel pump and the fuel pump wiring harness also. That seemed to make mine run smoother

Have you checked the fuel pressure regulator to see if its leaking?

I’m happy to assist further over the phone at https://www.6ya.com/expert/jeremy_d728a59f986299fa

I Have a couple of questions. First question does engine surge when idleing cold? 2. Does engine start quickly without a lot of cranking, Cold or warm? 3. Do you have funny smell at highway speed, or after you stop? With out this info and your imput, I would only be guessing. I am retired mechanic of 25 years. and would like to help if u need me.

the problem you are discribing would lead me to belive your fuel filter is clogged which is a simple fix 3 dollar part at any part store or on the worse hand your fuel pump itself could be weak you can open the gas tank cover and have another person turn the truck key to the on position without going far enough to start you will hear a faint whining noise if it is going bad or a louder noise if it is fine Good Luck

DTC Number
P0160 DTC Name O2 Sensor Circuit No Activity Detected Bank 2 Sensor DTC Number P0300 DTC Name Random/Multiple Cylinder Misfire Detected

bad MAP sensor or throttle body base gasket is leaking also the idle air valve ports can be clogged and the idle air solenoid is weak or bad

give a good tune up. if it still is acting up let me know

ICM idle control module, loc throttle body, it may be dirty, remove clean or replace, its easy..

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