So some of you reading this might have responded to some of my old posts on this same situation. But I now have new parts to add to the story. So the basic break down, my truck idles bad. Feels like its loping, not quite missing, but just a shake every few seconds. So far ive been throwing parts at it, o2 sensors, spark plugs, wires, air cleaner, most recently a fuel pressure regulator and tonight a fuel filter. Cleaned TB, IAC valve, MAF, seafoam through fuel system. About a week ago before i changed the regulator i measured the fuel pressure at 52 psi. which is too low (range is 55-62 psi static measurement). So i replaced the regulator, no change at all. Measured pressure tonight at 52 psi static (with just ignition on engine off) When i start the engine the fuel pressure drops to 48 psi. which blows my mind all together. Idle is sitting at a constant 500 rpm's. The thing that stumps me, is when I accelerate she clears up and runs just fine. No hesitation, stumble or surging. Only at idle in park, reverse, neutral, or drive. She drives down the highway at 55mph just beautifully, stomp it, she downshifts and goes. I recently had a flowmaster 40 series welded on, and while idling, you can hear the hesitation in the bad idle from the rumble of the muffler. Anyone have suggestions? I seriously doubt fuel pump. oh yeah btw 1999 silverado 5.3 w/ 86,000 miles.