Just bought a '92 4.0 with 171k miles on it. My oil pressure is pretty good when I start it and it idles at around 60 psi. But when it warms up, it drops to around 30 psi. I've not changed the oil in it yet, but the oil looks fairly clean and I wouldn't think it's worn enough for the drop to be completely related to thermal breakdown. Is my pump getting weak? What pressure is "normal" when it reaches operating temps? If it's on it's way out, I definitely wanna catch it sooner than later.

 

30 psi at idle fully warmed up is good, however you cant always go by what that gauge reads, the best way is with a oil pressure test gauge at the test port. However, if it never goes below that then you have no worries.

 

Newbie question......where is the test port located?

 

Base of the oil filter. Thats where the pressure sending unit is. Unscrew the sender and screw in a mechanical gauge there. Replace oil pressure sender with a new one because it will break when you remove it.