Hey guys can you help?
I have a 2000 5.3L Z71 106k miles. In the mornings when I crank my truck for the first time of the day I have a rough idle and then when I put it in gear and apply the throttle it hesitates (stumbles) for a few seconds and then it will go. For a few minutes it will still have a rough idle but after engine temp comes up it runs fine the rest of the day, even after 8+ hours being still while I work then problem comes back the next morning. It was giving me a check engine light and had codes like Misfire on cylinders 3,4,&5 I think is what GM dealer told me, also had code that O2 sensors reading rich.
Hear is what GM Dealer did: Replaced Fuel Filter, New Plugs, Cleaned Throttle Body, Loaded latest software into the ECU. Checked MAF sensor and ran two hour diagnostic test on truck and found no other codes/problems. They cleared the codes and two days later same problem. Took it back and they say still seeing O2 sensors reading rich and still getting several misfires on the register.

I read over the forum and from some other threads I went ahead and checked and no raw fuel in the hose going to the left valve cover. Octane does not appear to be an issue.
Any ideas?

Did they check the distributor? Did cylinder 6 also have a misfire? I'm asking because the firing order (at least on the 350s, but I think all the V-8 engines were the same) is 1-8-4-3-6-5-7-2. 4,3,6,5 are on the same side of the distributor cap. Worth a look at least, just seems odd to me that the misfiring cylinders would be on the same side of the dist. cap.

Did they check the distributor? Did cylinder 6 also have a misfire? I'm asking because the firing order (at least on the 350s, but I think all the V-8 engines were the same) is 1-8-4-3-6-5-7-2. 4,3,6,5 are on the same side of the distributor cap. Worth a look at least, just seems odd to me that the misfiring cylinders would be on the same side of the dist. cap.

Actually the newer systems dont have conventional dist. they have individual coils (8) 1 per cylinder. The firing order is the same as you stated.

No distributer on this one like SKIPR noted. They still have the truck and said it wouldn't do it when they kept it in the shop all night. The service manager asked if he could take it home and clear the codes and take his reader home with him and see if it would do it again on the way to work (going back to GM shop). Said he would watch it "real time" and hopefully see it when it occurs. They think it is the O2 sensors, maybe, when it is cold 50 degrees or less and that once they warm up from the exhaust temp they run fine the rest of the day. The one real positive note is that the service manager is interested in helping me find and cure the problem and willing to go the extra step and take it home and try to make it act up so he can see it!! Can't fault them for effort! Any other ideas or anyone else had a similar problem?

Here is a thought. On the drivers side fuel rail is the fuel pressure regulator. Pull off the vacuum hose and see if there is gas in it. If so replace the regulator. My truck had the same problem as you are describing. Good luck.

Check the basics prior to Electronic Systems. Ensure you don't have a vacuum leak somewhere

I would think another possibility would be the dreaded fuel pump

Same problem here... I replaced the fuel pump to no avail, also checked for vac leaks and fpr problems. I am getting SES codes lean on both banks. I saw a fellow over at pt.net that was having the exact same issue, and it ended up being rolled intake gaskets. A few others chimed in saying they had seen rolled intake gasket problems as well. I am replacing mine this weekend so I'll let you know what happens.

Actually the newer systems dont have conventional dist. they have individual coils (8) 1 per cylinder. The firing order is the same as you stated. Firing order on the new chevys are diferent, I dont have the book with me but i beleive the new order is 18736542 dont quote me on the order

I have been trying to figure out this exact issue for the past several months, and I think it finally came to an end this weekend. I went through new fuel pumps, fprs, etc and it finally came down to intake gaskets. I had read about a guy on another board having this problem, he took his truck to the dealer and they ended up fixing it with new intake gaskets, so I took the chance, heck its only $35 for the gasket set. When I pulled the old ones off, sure enough the rubber had sperated in a few places and left 1/8 inch gaps. Installed the new gaskets and everything seems to be back to normal.

Just to be clear, my truck would stumble on first start of the day, then run normally after it warmed up. It was throwing lean condition and misfire codes on both banks. It was getting annoying trying to leave in the mornings with it trying to die as I would take off.