1990 4.0 Renix. I don't know if this is a sending unit issue or a gauge problem. When I start it up, it quickly climbs to a nice 40 psi, but when I turn it off the oil pressure remains at 40 and by the next morning it will be 0-5. Where should I look first? I've never dealt with too-good of oil pressure

 

Are you looking at the stock dash gauge? If so I would suspect a bad sending unit. It's a cheap place to start.

 

I agree with Steve. The problem is likely the oil pressure sending unit, located right by the oil filter. It is a very common failure on the 4.0 liter Jeep engine.

Do yourself a favor and install a genuine Mopar unit from Jeep for the best accuracy/reliability/mating with your factory oil pressure gauge. Worth a few extra bucks.

Good luck and keep us updated!

 

I've also see the sender and galley to the sensor plugged up with sludge can similar slow response.

 

Well depends on your definition of "cheap".... cheap by new car standards?... absoFRICKINGlutely.... but I just replaced mine with a quality one and it ran about 65 bucks.
So while not a dirt cheap 10 dollar part.... at least it's not as expensive as say blinker fluid for a brand new Mercedes. LOL