Sure sounds like a vacuum leak at least at first. Have you checked for one yet? Not sure if they are common on those engines but just had a ford 4L have a vacuum leak at the manifold leak only cold for the first 30 seconds and then would run fine. Fuel trims were -40 and counting for the first 30 secs then went back to zero. After doing the intake was fine. Was surprised how fast the leak sealed up but showed up after awhile by smoke testing the manifold. And had no codes.