I have a 1999 Jeep Cherokee XJ and I've encountered a problem I've never seen before. I understand that the dash oil pressure gauge is not always reliable, but they usually in the ballpark, at least. About two weeks ago, my oil pressure gauge dropped while on the Interstate from around 40 to zero, then it went back to 40. A little after, it began to fluctuate all over the place. I figured right away that it was the oil pressure sending unit. I went to NAPA and bought a new one and put it in. Afterward, my oil pressure gauge went to just under 30 and sat there, no matter cold, hot, idle or high RPM's. I figured that NAPA must have given me a bad gauge, so they exchanged it for another new one. After a quick install of the second new sending unit, my oil pressure gauge again went to just under 30 no matter what, just like the first new sending unit. It's been a couple of days and the gauge just sits just under 30, always. Everything else looks good. The oil level is good. There are no unusual sounds from the oil pump nor are there any leaks.

Any thoughts?

 

My advice is to get your hands on a mechanical oil pressure test gauge and verify the oil pressure before you do anything else.

 

Good idea. I will pick up a manual oil pressure gauge tomorrow and report the results.

 

Sounds good, chief. Check it when it's at idle cold and observe the reading. Let it warm up and observe the reading. Then, raise the rpm's with the throttle linkage or the help of someone else and observe the reading.

 

Before going out and buying a mechanical oil pressure gauge, I was thinking, maybe, just maybe there might be a wiring problem somewhere. One might not suspect a wiring problem as the sending unit plugs into a wiring harness. I took a good close look at the wires as they head back to join the harness. As I looked closer, I discovered that a previous owner, for some unknown reason had cut all three wires, then they were taped them together again. As I looked, I spotted the problem, one of the three wires (an orange one) was dangling free. I suspect that when I put the first replacement sending unit on, I likely tugged at the wires causing the one to separate. The electrical tape around the old splice was brittle, so I removed it from all three wires, re-twisted the wires together, then taped them with new electrical tape. That did it. The dash gauge shows the oil pressure the way it should be, 50 at cold idle, 55-60 on the move and 40 at warm idle. Thanks for your help!

 

Do yourself a favor, go back to that splice, remove the tape, solder it, and use heat shrink. You'll thank yourself later.

 

X2 on this.

Glad you discovered the issue though.

 

Good idea. I'll solder it and use the heat shrink wrap. Thanks for the advice!