Hey all, been hunting down a low oil pressure issue on my XJ for a bit now. I bought it with a cracked head but it ran fine and didn't have any milkshake when I fixed it (replaced head/hardware).

Ever since replacing the head it's run like a champ. It's needed a couple sensors here and there (TPS, IAC and CPS) but the engine has been strong and healthy. That was confirmed by a compression test not long after a couple cycles of oil to clean out the block. Each cylinder was at spec.

One issue ever since then is that the oil pressure on the gauge would be fine at start up and while driving for a bit. But once it'd heat up to operating temp, it'd sit right at the first line (10psi) at idle and only go up maybe another 10-15PSI under load. Considering it had a cracked head and coolant could have ruined the bearings, I've kept a close eye on it. There has been zero noise/ticks out of the engine other than a bit of an exhaust leak because the previous owner omitted 2 exhaust manifold bolts.

Either way, to be safe rather than sorry, I've tried replacing the pressure sender but I've only tried some non-oem/mopar versions. Neither of them worked. Before spending the money for the $90 mopar version, I tried a mechanical gauge. Granted, it was a cheap one. It was reading near zero at hot idle... Thinking it was faulty, I returned it and got a bit pricier of a mechanical gauge (off amazon) and while it wasn't zero at hot idle with this one, it was at maybe 3-5 psi and 20psi under load. It's still possible the gauge was cheap/subpar but I doubt 2 gauges would be that wrong, especially since the second one had a relief valve to remove air out of the hose/housing.

Considering the potential of my bearings going out, and seeing what looked like very very fine glitter/metal in my oil, I sent a sample off for analysis at Blackstone. To my surprise, nothing came back too worrying! Definitely no major wear metals that could indicate bad bearings (lead/copper). Here's the report:


So, if all looks good there then would my next step be replacing the oil pump? I do have a slightly leaky oil pan gasket. And my valve cover leaks a bit more than that. Could that cause low oil pressure at hot idle? Should I just go ahead and replace the oil pump for peace of mind since it's cheap and I need to do my oil pan gasket anyways? Anything concerning on my report that stands out? What is the molybdenum from?

Side note, I've switched over to some 15w40 diesel oil and the XJ lovessss it. It's been running even better, and oil pressure has been up as well.

 

This is my third post in this forum in the last 3 years and I've yet to receive a singly reply to either of them. Is this place just dead? Or is there just not enough people that know about these engines here? Seems this forum has no value...

 

That's the spirit.

 

I think expecting an answer in 2 hours might be over doing it..... Patience has value, try to have some.

However, my bet is on bearings over the oil pump. You only had 600 miles on the oil for your test which is next to nothing and wont give you good results. On top of that, its contaminated, with fuel, in just 600 miles. Thats bad. Fuel kills the rings and bearings pretty fast. So you have 2 different issues going on. Fuel getting past the rings and worn bearings.

BTW - blackstone does not actually test for fuel in the oil. Its a general assumption based on the flash point vs viscosity.

 

Fair enough. Last vehicle related forum I was involved in was for a volvo s60r and it was a very active forum with replies right away. Sorry for the negativity, haven't had much luck getting any help here until your response.

So I've wondered about that. It's got a trace amount of fuel, but I'm sure that'd add up to a higher amount by 4k miles. If they're not testing for fuel, then it might be possible that's coming from marvel mystery oil? It has a flashpoint of 160F according to someone who did an analysis of straight MMO through blackstone. MMO's own safety datasheet has the flashpoint at 128F. Given I had about 1/3 of a bottle in the oil that was sampled, maybe that was it. But I do have some after market/questionable 4 port injectors that could be faulty. Maybe I can see if the rail holds pressure after the pump is shutoff? And I did a compression test but not a leak down, maybe I can add that to the list.

Was that the only reason you suspect bearings are bad?

 

It was recommended to me by the local engine rebuilder and it pretty much aligns with how I've operated. Most any time the oil pan comes off, if it's been a while, replace the oil pump and rod bearings.

I've also come to the conclusion that it's so super easy to pull the motor in a XJ, when in doubt just do it.

Following the theme of the first comment, any time the engine is out of the vehicle, if it's been a while, I'll replace the freeze plugs, rear main seal, main bearings and timing chain. Good ol' shade-tree junkyard rebuild...

 

Your issue is common after the head cracks. Coolant wipes out the bearings. Mostly the cam bearings for some reason.

 

See that's what I thought and why I got the oil analysis. But I have no wear metals present in my sample... So either the cam bearings/bushings are not wearing, or they're so worn they're not showing wear metal in the analysis... which if that was the case I'd be having much larger issues right now. So while it could most likely be the cam bearings, why would no wear metals show in the analysis? It's not adding up.

 

The easy route would be to drop the pan, replace the oil pump (no need for a high volume pump, just standard oem type is fine) throw in a new rear main seal, pan gasket and put it all back together. If it still runs fine, and oil pressure is ok...run it like you stole it. If you have low or no oil pressure then you are going to have to go into the motor and replace bearings and such, or drive it till it dies.

I am not a professional mechanic...
Mostly just under the shade tree....
Hope this helps...
have a great weekend!

 

That's certainly more my style and what I was planning to do at this point. Since the oil analysis doesn't show any concerning wear metals, I'm not just going to assume it's the cam bearings even though there's a high possibility it's that considering the history of this vehicle. A $60 oil pump, $80 pressure sender (current one leaks oil through to the connector), and $40 or so in gaskets/seals. Couple hundred to rule out all but the cam bearings, plus a fun afternoon wrenching on the jeep, seems the way to go. Time to get those parts ordered, thanks!

 

A sign that bearings are at their limit is oil psi while cold is ok and close to none while warm. It could also be that the oil pumps relief valve is stuck open but I doubt it.

I have never used MMO so I can't really comment. Although, that is a low flash point and could contribute to blackstones fuel reports. Did you note to them you were running it? If you have china 4 port injectors, that very well, most likely, is the issue.