Ok, i have been having this problem for a good 8 months, and no one can tell me what wrong. After driving and turning off the truck, when i get going again 30-45 minutes later it is a total turd for like a minute, then it drives like normal??? It is worse in the winter when its cold and even sounds like it backfires if you put the pedal to the floor. One mecahnic said its the cats, took it to the dealer and they said, "not the cats, and nothing is wrong with how it is driving." b.s. :mad: . And i paid for them to tell me that. Anyways, its nothing with the engine because unfortunately had to replace it due to rod knock. What electronics or other components could be causing this? Something with the automatic timing? Im lost on ideas and need it fixed, its hard pulling out into traffic with a trailer and having no power.

Have you checked the filters and fuel pump?

Fuel pump or cats? Got to be able to catch it in the act for someone to tell you something is wrong.

Have you checked for any codes, ie bad O2 sensors? O2 sensors need to heat up a bit before they work if they are covered in carbon or are going bad. The code scan may point you in the right direction. Also the o2 sensors are located in the exhaust and might not have been changed with the motor swap.
JP

Ok, i have been having this problem for a good 8 months, and no one can tell me what wrong. After driving and turning off the truck, when i get going again 30-45 minutes later it is a total turd for like a minute, then it drives like normal??? It is worse in the winter when its cold and even sounds like it backfires if you put the pedal to the floor. One mecahnic said its the cats, took it to the dealer and they said, "not the cats, and nothing is wrong with how it is driving." b.s. *******:mad: . And i paid for them to tell me that. Anyways, its nothing with the engine because unfortunately had to replace it due to rod knock. What electronics or other components could be causing this? Something with the automatic timing? Im lost on ideas and need it fixed, its hard pulling out into traffic with a trailer and having no power.

Somehow, I believe you have a 5.3, not a 5.7. With that said, unplug the vacuum line from the fuel pressure regulator on the driver's side fuel rail on the engine. If gas pours out, or you simply smell gas in the vacuum line, the regulator is going bad. Very common on a 5.3, but doesn't exist on a 5.7.

I think they still used 5.7L on 2500's and 3500's. They did not put the 5.3 in those trucks. They changed the body style for 2500's in 99.5 and those had the 6L.

Somehow, I believe you have a 5.3, not a 5.7. With that said, unplug the vacuum line from the fuel pressure regulator on the driver's side fuel rail on the engine. If gas pours out, or you simply smell gas in the vacuum line, the regulator is going bad. Very common on a 5.3, but doesn't exist on a 5.7.

The 5.7 does have a regulator it is mounted on the spider injector. It does sound like a regulator letting fuel run back to the tank.

This is a old style carry over truck, so i do indeed have a 5.7. The dealer said there were no codes, and that the co2 sensors were within the parameters. Also said that he cats were not bad, did a pressure check, and also said that if they were bad, the truck wouldnt run for crap anytime. Fuel pump was replaced about a year ago, fuel filter, plugs, wires, rotor, cap are all new.

I will look into the fuel regulator idea though. As for the o2 sensors there are 4 of them one in front of the cat and one behind, and there are 2 cats. HORRIBLE exhaust setup, thanks GM. THey are like 88 dollars a piece and dont feel like guessing at stuff that cost that much when the dealer said they are fine.

Just for grins unplug all the O2 sensors & see if it still does it. Could be one that makes it go a little lean till it gets warmed a bit. THe computer will go to a preset fuel curve if the O2's are offline & it might run better. If that changes the running then you can hook up the scanner & watch the fuel trims more closely & figure out which one is the problem. Don't know if this will work but would not take long to try.

Clean the MAF with a Q-Tip & brakekleen also, The MAF can do this also.

This is a old style carry over truck, so i do indeed have a 5.7. The dealer said there were no codes, and that the co2 sensors were within the parameters. Also said that he cats were not bad, did a pressure check, and also said that if they were bad, the truck wouldnt run for crap anytime. Fuel pump was replaced about a year ago, fuel filter, plugs, wires, rotor, cap are all new.
I figured it was a carry over, but I had to start somewhere.
1. Verify that the EGR valve can do a full open and close (I use a Snap-on scanner). A binding EGR usually shows up as occasional ultra-firm 1-2 shifts on an automatic.
2. Verify cylinder misfire counts (Snap-on again.) This will tell you if your new cap and rotor are wothless or not- some aftermaket ones corroded way early.
3. Verify IAC movement (clean the passages while your looking.)
4. Ensure air cleaner is installed correctly (the elbow to the MAF should not pull out of the element with the lid still latched).
5. This should maybe be Number 1. Does the computer know how hot the engine is? Test or change the CTS at the gooseneck elbow. What if the computer doesn't know the truck is cold???? The computer cannot use the data from the gauge itself (and the gauge uses the sender in the left side of the block).

Good 'ol 'R' engine. My question is: Do you have an extended crank when starting?? In other words, does it take longer to start than it use to? If it does, than it's the Fuel Pressure Regulator.

It starts right up, idles fine, only know something is wrong when you give it gas to go.

The 5.7 does have a regulator it is mounted on the spider injector. It does sound like a regulator letting fuel run back to the tank.
Will want to make sure the "spiders" are not cracked and leaking fuel also, making it misfire and lose power.