350 vortec engine runs rough when warm

[Dark Knight]

Hey everyone, I'm trying to help my buddy out. He has a 99 GMC (obs) with the 5.7 vortec. His truck just died on him going down the road one day and he asked if I could fix it for him. So far, I've replaced the cap and rotor, crank and cam sensors ( due to a cam/crank correlation error code), re-timed the engine per GM tech manual, and replaced the fuel pump. The code never came back once erased. We have good fuel pressure at 58 PSI, I thought that was a little low, but I think it's enough to run. The thing I can't figure out is that when the truck is ice cold, I can start it up no problem and it purrs like a kitten for about 10 minutes. Then, it slowly starts to sputter, then shake and backfire when we try to press the accelerator. It gradually gets worse until it eventually dies. If you crank it back up, it runs the same way. Let it sit for a couple of hours, crank it back up, and it runs great for a little while, then runs crappy again. It has to be fuel system related. I'm thinking maybe the EGR valve is opening too soon or it's flooding out (bad fuel pressure regulator?). My buddy said it threw a code when he ran it earlier, so maybe its something other than the crank/cam correlation error. I'm going to check it out tomorrow again. Anybody have any ideas on what this problem might be? It just gets me how it runs great at first and then slowly dies, let it sit for a while and it's fine again. I can fix most anything else, but this thing has me stumped...

[speedygonzales]

chances are good it's a closed loop issue. Try disconnecting one of the front O2s (to force it into open loop) and see if it runs better once hot. If it does run better (not the best but better than in closed loop) it could be one of the front O2s are bad, the MAF is dirty and a few other things.

Incidentally the front O2s are considered a tune up item. So is cleaning the MAF.

Could also be a "hot" issue like a coil or the ICM breaking down after they are hot, but look for other things first. The coil you could look at in the dark for what I call "side firing" issues where the spark jumps from the body of the coil to the side laminations. And just as before, coils are considered a tune up item as well. If you remove the ICM you will see there is no longer heat sink compound left on the back of it, if it has not been serviced in a long while as well.

I still think however you have a closed loop issue.

[tucktuck]

I had this same exact problem on my jeep wrangler about a month ago. I am pretty handy and fix and maintain my own house and cars....but this one stumped me BAD! I thought it was a spark issue so I tuned it up...plugs, wires, cap, rotor...nothing changed. Ripped the throttle body apart and cleaned it...nothing changed. Put a new dizzy on it (that's what the forums suggested) along with a couple cheap sensors. Nothing! Checked the fuel pressure and it was good. I finally broke down and took it to a jeep mechanic. He played with it for a couple weeks and figured it out....fuel pump. It wasn't working once it got hot. Fuel pump never crossed my mind. I always thought that when a fuel pump went out...it just quit.

[Dark Knight]

Got it solved. The "new" crank sensor that I installed was faulty. The first dang part I put on too. Could have saved myself a lot of time if I knew that was the deal. Just goes to show you that new parts aren't always good parts. And it was made in the USA too! :/