know I'm new an butting in, but I just cant resist.
If you want to install a manual oil psi gauge, here is what you need to do.
#1. Go spend $25 and buy a manual at the local parts store.
#2. look up the oil psi sending unit in the manual. (this is the little thinggy that the wire for your stock gauge hooks on)
#3 Go back to the parts store and purchase a mechanical oil pst gauge (and all the little parts that go with it to make it work. ie line (might be brass or nylon, etc...
#4 Drink 3 or 4 beers and a couple of shots of crown to make sure you have the sufficient combination of courage and mechanical aptitude for the job.
#5. Decide where you want to drill holes in the dash to mount the gauge. (the gauge should be mounted, not you mounting the gauge)
#6. Unhook wire to sending unit.
#7. unscrew old sending unit. (good job, so far, have another beer or 2 for pre-success celebration)
#8. Route the new oil line that Im' sure came in your kit and will be long enough. from the back of the gauge, to the spot where the old sending unit used to be.
#9. Invent some new swearwords while you discover the line in the kit is either, kinked, too short, or just plain hard to put where you want it.
#10. Install fitting for the oil line in place of the old sending unit. (It did, come in the complete kit right).
#11. Install oil line in fitting and to back of gauge.
#12, You deserve another shot of crown.
#13. ah, luck 13. Check to make sure the oil line is connected, free from hot thinggys like exhaust, free from moving thinggys like doors and hoods.
#14. Fire up the motor and look to see A) how much oil psi you have and B) check for leaks.
Just a note of caution. If you have an oil leak there, you can pump your pan dry in less than 5-10. This tends to have expensive consequences. So I recommend you check and recheck your work.
Now on to your original question. The oil pump is in the same place as all the other 4.0's
It's inside your oil pan. It's usually a pain in the ass to change. Sometimes you can shim an oil pump to get more psi. If you really have lost psi it means your motor is getting loose tolerances (worn motor). The way to fix that involves taking the motor out and completely rebuilding it.
TTFN