02 ext cab 4x4 originally had a 4.8. Original engine was going out so I decided to upgrade to a 5.3 if I was going to replace it. That alone was a nightmare. The place I got the engine from sold me 4 bad engines before I got a good one, having to go through the warranty process each time. Now I have a seemingly good engine. I replaced every sensor, intake, IAC, ECM, gaskets, water pump, radiator, hoses, injectors and most of the vacuum system. Also changed it to a self regulating fuel filter and deleted the vacuum fuel pressure regulator and return lines. The issue I have now is that it will throw knock sensor codes now and again and also random misfire codes. I have replaced the sensors and harness twice since the engine swap to make sure I didn't get bad parts. Also, when I cold start the truck it idles a bit high, but other wise drives fine. If I shut it off and restart it, it shoots up to 2500-3000 rpm and won't come down. I have to let it sit for hours before it will start and run somewhat normally. Most of the time, it will not start on the first key turn. It takes 2 to 3 tries and it almost always backfires a little even when stone cold. When I check the spark advance, it shows 37-43° on cold start at idle. I did trace down a few ground issues and corrected them. I have completely replaced the entire engine wire harness, main fuse bus and tried 3 different ECMs and always have the same results. The only sensors I have not changed (2 or 3 times in some cases) are the O2 sensors. The high idle happens on second start weather the truck has ran for an hour or 30 seconds. I have even tried turning the VATs off and running it with the body control module disconnected in case it was bad, and no change. All sensors test OK except the crank and cam sensors as I have no idea how to test them or if you can. It only gives the random misfire code when it goes to the high idle. It has been smoke tested, with no leaks found. The only thing I can think is that there is an electrical issue in the cab/body somewhere that is causing a voltage issue to the ECM. I have been working on this truck for 7 months and trown a ton of money at it and still can not figure it out. I also can't get another mechanic to touch it. Any insight or suggestions would be appreciated as I and my mechanic are at a loss and have no idea where to go from here. Our next plan was to install a stand alone wire harness to isolate the ECM and engine from the rest of the truck to see if it runs correctly then. Thanks for any help.