I've got an E350 with a freshly tuned up 460 big block. It runs great in open loop but as soon as it goes into closed loop it misses. If I colapse the vacuum line to the MAP sensor causing the CEL to come on and I'm assuming forcing the computer back into open loop it runs great without a miss. Do I have a faulty computer or a bad sensor that isnt tripping a CEL? RB

 

I would disconnect the battery for a minute or so, reconnect, and then drive it through a complete cycle. I would pull the codes and see what you get. Make sure to read the memory codes also.