Low/No Oil Pressure at Idle When Hot

DD, 99 XJ, 4.0, AW4, NP242, 208K, & about 4,000 miles since the (probably original) head gasket was replaced by a trusted shop.

A couple of weeks ago, on may way to work (~25 miles each way) I noticed the oil pressure gauge was reading really low when I was at a stoplight. It had been lower than "normal" since the head gasket replacement, but I was also running 5W-30 after the head gasket, since that's what the shop recommended for at least the first 1000 miles. Within a few minutes, the gauge would drop to zero and the "Check Gauges" light would come on any time I was at a stop & idling. If I gave it just the tiniest bit of throttle, it would come back up, but still around 15-20 ish most of the time. Once I was moving again, pressure was back to the usual range, but it dropped quickly every time I stopped. Added some STP High Mileage Oil Treatment to see if it would make a difference, and it seemed to do the trick. Until last night...

Past weekend I changed the oil, this time with 10W-30 QS High Mileage, which I've used for years, and filter, Mopar 090 to replace the generic one from the shop. Didn't add anything else, and didn't seem like I needed to. Ran perfectly fine with great oil pressure until last night. Now it's doing the same thing it was before. I added a can of Restore/Engine Restore/whatever it's called, since I'd used that before on a previous XJ and seemed to have good luck with it. It didn't do much, if anything. After 10 min of city driving this morning, in 36* temps, I was back to the same issue. Made it to work, but was trying to figure out if the engine was making more noise than normal (or if I was just freaking out) pretty much the whole way there. Haven't tried the STP yet, since I was hoping the Restore would do the same thing, but it didn't.

To be clear, the issue only seems to arise when the oil is hot, and only at low idle. Oil pressure has been great since the STP, and wasn't any different at first after the oil change.

I have access to a manual oil pressure gauge, just not easy access to it. Nor do I have time to run the test pretty much any day this week, and I really can't miss work this week either.

I'm PRAYING that this isn't a sign of expensive engine repairs coming soon, since I just dumped a ton of money into it in December. But I'm also not sure where to go next. Help!

-alopeks

You definitely need to put a mech gauge on it to verify whats happening. After that, if it reads the same........

the senders are known to fail, mine did at 206K. but agree, would want to know more about what pressure it's actually running with before driving it.

With the old head gasket, was it dumping antifreeze into the engine? It may have junked the bearings.

I would suspect that the oil pump is just getting old. I had low pressure on my 212K '90 but once I replaced the oil pump I was right back where I was supposed to be.


Definitely test the sender to remove that from the "suspects" list but don't be surprised if it is time to upgrade the pump. The good news is that it isn't a difficult job but you should plan to spend some time on your back.

If you are running a FRAM filter, trash it and get a decent brand such as WIX. I have seen multiple engines with this problem due to the FRAM filters.