imjakethesnake

So I am having a slight oil pressure issue... Ive already replaced the pressure sensor, oil pump (high volume), and inspected the bearings. Also cleaned out the brass elbow. My pressure starts at 50psi cold, but after 20 minutes of driving, it drops to around 11psi hot idle. 35psi while driving. Also running an oversized purilator filter with 10w40 and a bottle of lucas. different weight oils seem to make no difference and no noises or issues have surfaced, but it just makes me paranoid.
My next guess would be maybe a clogged oil galley or something. The oil STAYS CLEAN even after running it for 3000 miles. I was curious if i had a clog in the port leading to the pressure sensor, so i took the sensor off and inserted a hose, cranked the engine and revved it (briefly). the oil that comes out of the opening is black, yet the oil in the pan is clean? Doesnt seem right... the engine has 200,000 on it and its a 1999 4.0. Ive replaced EVERY SEAL replaced the head(new), oil pump, sending unit, my mechanic checked the bearing clearances and said they were perfect, and he thought i may have a clogged oil galley somewhere.
When i replaced the oil pump and pickup tube, the pickup tube looked clogged and there was muck in the bottom of the pan. when i saw the old oil pump i thought for sure i fixed the problem, but sure enough after driving for 20 minutes it slowly started creeping back down. Also, Ive tried everything from 5w30 to 10w50 oil and the pressures in the same range every time. I would think there would be a pressure difference with different weight oils...

I guess my question is...
What am I doing wrong here?

Thanks guys!

bigbadon

Drive it. At 200k sounds normal. You do not have a problem. 11 psi is way more than needed to keep the lifters pumped up. I have 4 - 5 psi at idle and 35-40 at cruising. Just because you have low pressure does not mean your not getting oil flow. A long as the oil is being distributed to all bearing surfaces you should be ok. How long has this condition existed

90laredoo

Well just be happy your not in my position.. I'm running 10w40 and a quart of Lucas and my oil pressure drops to 0 At idle when it warms up. Hopefully a stroker engine is in the near future for me. Haha

Bustedback

Pull the oil fill cap off and run the engine, take a look and see if there is oil splashing around inside the valve cover. The best thing to do would be to put a manual gauge on the engine and see the true oil pressure, you could have a goofy gauge in the dash.

tjwalker

Assume that you hopefully "verified" your actual oil pressure with a mechanical gauge? If you didn't, you must do so. there can be some inaccuracy between the sending unit and the gauge. I know you replaced the sending unit, but gauge inaccuracy, although kind of unusual, is certainly something you must consider.

If you have a "verified" 11 psi at hot idle, I agree. Drive it. I have seen many, many 4.0 engines run for tens of thousands of miles with around 10 psi.
Doesn't make a lot of economic sense to rebuild a motor that likely will continue to provide good service.

mudboy

10 psi for every thousand rpm is an old rule of thumb.

413maxwedge

If you read your haynes repair manual it would tell you at, 600 rpm= 13 psi --1600+ rpm=37-75 psi. Page 2d-3 of YOUR HAYNES REPAIR MANUAL. IT's fine run it.HAYNES BOOK'S are only 25 BUCKS.

goneboating06

Sounds pretty normal to me, especially with how many miles you have. You had your main bearings inspected, and they were all fine so obviously your getting oil flow to them. Just run the engine until it gives out. Theres still a lot of life left in it, go out and enjoy it.

imjakethesnake

It has been verified by 2 mechanical gauges. also, i have a mechanical gauge spliced in to the sensor as a permanent fixture. (See below)
And i do see oil inside the valve cover when running. When i changed out the oil pump, there were some small bits of metal in the sludge, but not a scary amount... Seemed pretty normal. ive seen alot worse when dropping chevy pans. But it did add to my curiosity. I guess i could just ride it out, but i was worried that eventually its going to drop to zero like i hear happens to lot of other people. I just wish i knew the history of the maintenance. I bought it used with a brand new head and valve cover. Makes me wonder why the head was replaced to begin with.... hmmm

bigbadon

My situation is similar,head replaced too. May have been cracked and coolant wiped out cam bearings which would explain the "no knocking" But as long as the pressure comes up above idle and no knocks I do not see a concern. The cam bearings can be checked by doing a air test with the pan off but if they are indeed bad there is not much you can easily do about it. So I plan to drive mine until I hear some significant knocking. Oil pressure is not that important as long as oil is getting distributed to all the moving parts. Pressure or the lack of at idle is an indication of increased clearances. Low oil pressure is NOT detrimental to engine failure as long as you have oil flow....a worn engine is detrimental to oil pressure though.

imjakethesnake

I did read my haynes manual and it was running a little less than the minimum stated above. Thats why I started the thread. My Haynes manual only cost me about 17 bucks. But i did buy it like 3 XJ's ago. Thist is the first XJ ive had where the oil pressure dropped below 30psi even during idle.

Current psi: Hot idle(700rpm) - 11psi, 1600rpm - 21psi, 2000+rpm - 35+psi.

So id say (when hot) it runs less than what it says in the haynes manual. Am i right?

tjwalker

Many XJs run well under 30 psi at idle. My 99 has run just a smidgen over 20 at hot idle since I drove it off the lot in 1999.

I think you are getting too hung up on the spec. Are you a bit below spec? Yes. Is it verified? Yes.

So your options are either to drive it or tear it apart. Driving it seems like the most sensible option as I don't see this engine going south anytime soon.

Moncheche

11 psi at operating temp is fantastic. Don't be tempted to through heavier oil at it.

imjakethesnake

Yeah I did try a slightly thicker oil just to see if it would make a difference, when it didnt, I didnt want to wear the engine any further so i replaced the oil and the filter right away with 10w30 and 1qt of lucas. I feel bad for wasting good oil like that but its ok. It gets recycled anyway.

imjakethesnake

Seems like my best bet is to just ride it out and continue changing the oil on a regular basis. And when this engine meets its demise, time for a rebuild. Thanks for the reassurance everyone! I'm hoping at least for 300,000 out of this beast!