I finally got a replacement engine installed into my 01 pathfinder, now alot of weird stuff has started hapening.

With the engine, if you rev the engine while in park, it will rev to 4000, but once it hits 4K it hesitates alot to go any higher. If you give it enough gas it will advance to 4500+ to 7K. The mechanic that installed the engine said there was a code for the coolant temp sensor. I was thinking that this could set the ECU into a failsafe mode/fuel map????

Now with the transmission, it will drive fairly ok in the city, but once you hit the higher speeds of the highway(about 70-75) it hesitates to up ****, and once it does after while, the car will shake then downshift out of OD. One of my friends told me this could be the lockup on the torque converter disengaging, causing it to downshift, and act weird alltogether on upshifting.

What im thinking, is that it is a problem with the engine for the most part, cause when the transmission downshifts at 70, it tries to rev higher at about 4000 rpms, which is where the engine is hesitating. Could this be it???

Any ideas???