I just finished swapping the 3.9L in my 2001. I picked up a used motor that had been sitting for a couple of years.
When I got it in and hooked up today It started right away but my truck has been sitting for awhile and my alternator had seized so I left the belt off.
Problem is that there is no oil pressure showing and speedometer went all the way over to the max and stayed there.
Motor sounds good with only slight lifter noise and no smoke, only ran it for two minutes kept revs under 1500 RPM and will try again tomorrow to see if pressure relief valve was stuck from having no oil all this time.

 

I checked the dash gauges by turning key on and pressing trip reset.
speedometer went back to zero when check complete.
I left the oil pressure sending unit in that was on used replacement motor and think that it might be bad I will try to put my original one in because I know it was working.
I started truck today and lifter noise is gone so sounds like oil is circulating.
and still waiting to change alternator so I can let it run and drive it until warms up.
when oil sending unit is out I might start truck and see if oil comes out of hole.

 

Just put the sender in from your old motor. The one you have now might have been for the wrong year.

Starting the truck with the sender out is going to make one big mess.

If you really want to test it put a mechanical gauge on it temporarily.

 

Truck is an 01 and motor I put in is an 03.
Truck was sitting around all apart for 4 months and the alternator was left out and uncovered so it seized and now waiting to get to junk yard for another one before I can put belt on and run it.
What kind of wrench is best to get down back of intake to remove sender.