So I have done some searching and can't seem to figure out what would be wrong here. I have a 99 GMC Sierra with the 5.3 and 88xxx miles.

This started happening a couple of weeks ago, just when it started to warm up outside. It seems that when I start the truck for the first time each day it turns over just fine. once I get to my first destination and park it for a bit it has trouble starting again for the rest of the day. I have to pump the gas and it takes 5 seconds or more to turn over. Even then I have to rev it a time or two to keep it from dying. Then every time I get to a traffic light the rpms seem to dip from 500 to 300 for a split second and then go back up to 500 and remain there until I start moving again. Come home and go to bed, wake up the next morning and it starts back up immediately.

I ran some Seafoam through the gas tank and it seemed to clear up for a day or two and then came back. I cleaned the throttle body and that didn't fix it either. Anyone have any guesses?

 

put a fuel pressure gauge on it and watch for pressure leakdown.