4.0 idles fine but can't handle load
Just had the engine swapped out on my YJ 4.0L. Everything was hooked up and ran it for 30 minutes according to specifications. When they took it for a drive accelerating and decelerating, again according to specs it was running great and just like new then all the sudden it started to jump and sputter and surges rpm's up and down erratically. The jeep started to act like it wasn't getting enough fuel and the last two miles it backfired and bucked like a mule, dying several times before they made it back to the shop. They ran codes Nothing, Replaced the fuel pump, cleaned the tank out and checked the fuel pressure with no luck. Timing checked and it was out 180 degrees but if set right wont even start. Change the O2 sensor, throttle position indicator, rotor and cap, spark plugs and wires, and had the cam sensor adjusted as well. Nothing. Put the timing back to 180 out and it starts, but once its under load it does the same thing. Jeep runs fine under idle, but as soon as you put it in gear and apply load all hell breaks loose. Jeep bucks, snorts, backfires, dies. Borrowed a ECU and still persisted, so they gave up on trouble shooting. Suggestions?