My wife's XJ had the oil pressure gauge pegged (80 psi) while on the way to the beach yesterday. I told her to pull over, let it idle, and it dropped to ~60 psi, which is still much higher than normal. Then back under load, it rose to 80 again.

I've read that oil pressure sending units are notoriously flaky in 4.0L 6's. I replaced the sending unit, oil pan gasket, rear main seal, and valve cover gasket ~2 years ago. Everything else was fine: no CEL, water temp was good, no smoke, no knocks or misses or strange sounds. Oil was ~1/2 qt. low, but looks OK on the dipstick (not milky).

Anyone know of any common causes of high oil pressure or high reading besides a bad sending unit?

Thanks!

7/10/2012 10:36:45 AM

Put in a real oil pressure guage.

7/10/2012 10:44:55 AM

^ do that, check it

still high...

spun bearing...

7/10/2012 10:55:55 AM

Check/replace your oil filter too.

7/10/2012 11:17:26 AM

check the wiring. make sure the connectors are clean with no corrosion. cherokee's are known for electrical gremlins. i had the water temp gauge do the same thing. it would peg randomly and other times it would jump around. removed the wire, cleaned and added dielectric grease, and hadn't had a problem since.

7/10/2012 11:51:05 AM

Thanks for the feedback, y'all. I'm overly anxious since she's at the beach for the week and therefore I can't take a look at it. I just hope it's sensor/wiring related and not actually high pressure due to a major problem. I'm anxious to check it with a mechanical pressure gauge so I can either calm down or start getting pissed at the potential for major issues.

7/10/2012 12:09:43 PM

At minimum I'd get her to do an oil change and ask to keep the oil filter before continuing to drive the vehicle. Just have them toss the filter into a few grocery bags so you can examine it later for defects or large amounts of trash in the screens. Could be a collapsed oil filter, clogged oil filter, blocked oil passage, etc. It might be time to run one of those detergent oils in it to try to clean all the little oil passages in the engine as well. They can probably do this at an oil change shop, although they'll undoubtedly overcharge for it. Still cheaper than repairing a damaged engine if she's on vacation and that's the only option.

See also "Causes of High Oil Pressure" here:
http://lubricants.s5.com/oil_pressure.htm

[Edited on July 10, 2012 at 2:44 PM. Reason : s]

7/10/2012 2:41:44 PM

bad sending unit. Mine did the same thing, replaced it and it worked fine for a while, now the gauge reads higher than normal again. POS sending unit from Advance.

7/11/2012 8:21:08 AM

why does a spun bearing affect oil pressure? j/w

[Edited on July 11, 2012 at 2:31 PM. Reason : not how]

7/11/2012 2:31:08 PM

Oil flowing through the tight clearances of the bearings and crankshaft journals is where the pressure builds. If a bearing gets spun or starts to wear notably the clearances get larger (allowing more oil flow) and the pressure goes down. It's no different than the pressure that builds up inside a water hose when you barely open the nozzle versus opening it all the way.

I just can't figure out how a spun bearing would make it go up rather than down. Maybe in the very earliest stages if the crank bearings are overheating and the metal expands, but that seems far fetched to me. Seems like you'd know as soon as the engine cooled and you cranked it again if that were the case.


[Edited on July 11, 2012 at 5:32 PM. Reason : s]

7/11/2012 5:22:38 PM

It's fucking gunked up oil or a bad sending unit. Spun bearing my ass she would hear it. Don't give me that being the reason why it's jumped 20 damn psi or so over normal.

7/11/2012 9:20:04 PM

Lol you are right when the bearing spins and the oil holes don't line up any more nothing ever happens to oil pressure... Nothing at all...


And no I didn't have a jeep that did this.

7/11/2012 10:10:37 PM

Did you take any beach trips with that spun bearing or did the horrible racket coming from the engine sway you.

7/12/2012 12:05:03 PM

No conclusion yet, but the behavior has changed and I'm almost positive it's just a sending unit. First of all, she got an oil change and then made it back from Oak Island with no problems and the gauge reading zero psi. I kept waiting for the call that she heard a big clunk or was billowing smoke or shot a rod through the hood, but alas no such call came through.

Then when I finally got to drive it the pressure was 0, then I'd come to a stop, let it idle and it would jump to 80, step on the gas at all and it dropped to 0; the complete opposite of any sort of "normal" behavior.

I'm going to replace the sending unit this afternoon after I throw a mechanical pressure gauge on it as a sanity check.

Thanks for the feedback, everyone.

7/16/2012 2:00:17 PM

As expected, it was just the sending unit. Mechanical gauge read 30-50 psi at various throttle positions (just in park, not under load). So, I replaced the sending unit and now all is well.

So, after <1 hour and a free replacement sending unit from Autozone I'm feeling good and I'm so thankful it wasn't anything else.

7/17/2012 10:13:27 AM