Low Oil Pressure Indication

Have a 1998 Jeep Cherokee 4.0L with 278,000 miles. Installed a rebuilt motor 45,000 miles ago. Normal oil pressure indications prior to installation of rebuilt engine. With rebuilt engine pressure was between 30-40 even when driving on highway. Mileage remains good and no overheating was observed on the dash gage. Recently oil pressure has dropped to between 10-20 with check engine light coming on at idle. Oil pressure indication from panel gage never rises between 20-25. When starting cold, pressure runs between 40-50 for about 10 miles then gradually begins to drop. Recently had to replace fan motor. Have replaced oil at regular intervals with no excessive loss observed. Have replaced sensor on engine block and low oil pressure indication continues.

I’d want to check the oil pressure with a bourden tube pressure gauge, no electricity required, to verify what seems to be happening. You should be able to remove the oil pressure sender to substitute a gauge unless this will give your vehicle’s computer, fits. If that it true, then use a “T” adaptor. You can view the gauge with the hood up, with a cold and a warmed engine if you don’t want to plumb the gauge to where you can see it while driving. This is a suggestion to consider as you have none so far.

PS, I don’t know if your vehicle uses a bimetal-heater type voltage regulator for dash gauges like our Ford did. It was erratic and the gas gauge, oil pressure gauge and temp gauges were all affected; went up and down at the pleasure of the dash gauge voltage regulator.

The engine sump may be running out of oil. Where would the oil be? In the top of the engine. This pooling under the valve cover could be caused by the block drain passages being partially blocked. Contrary-wise, the oil pickup in the sump may not be picking up properly. It may be partially blocked, or, its gasket may be leaking. Dip the oil level before and after the oil pressure indication falls. ---- Chrysler has a low oil pressure engine shut off. So, you would have to T the test oil pressure gage. The oil pressure sensor is maybe a 1/8 inch npt fitting.