I have a 1999 5.4 liter expedition the motor went out on us, so we had it replaced with one from a 2003 expedition 5.4L with 65k miles. Since then, when I give it gas it seems to accelerate very choppy and slowly, unless I get the rpm's really high, and then it run just fine. The check engine light is on and indicates a Oxygen sensor is faulty. But I didn't think that an oxygen sensor would cause that much trouble as I have had numerous vehicles with o2 sensors out and ran them with no problem. I did so far replace the fuel filter to no avail. And the fuel pump is also only around 4 months old. One thing I did notice is that towards the rear of the vehicle you can sometimes smell gasoline.

 

My SWAG would be a vacuum leak, but it would be helpful to know specifically which DTC code(s) are being stored, and which computer you are using - the '99 or the '03.

 

So...you figure they added O2 sensors into the design just for fun?

You most likely have a COP acting up, but I'd still take care of the O2 sensor since it handles the fuel mixture.