Oil Pressure - thoughts or advice??

[Ecomike]

I would move on to 20W50 at this point. I have never tried it, but I have heard good things about Mobil 1 15W50.

I will ask again, what is the cold, oil start up pressure?

Some one poster here once that they were using 80 Wt gear oil in their 4.0, that was still running, but had lots of miles on it.

 

[ChrisR]

cold start psi is 40-45 psi

 

[Ecomike]

cold start psi is 40-45 psi

Then the oil pump suction screen may be clear, or pretty clear. And the pump can obviously deliver pressure.

 

[Jeepguy03]

The rule is 10psi per 1000 RPM. I'd run 10w40 or 10w30 if you think it runs quieter with 10w30. Use a good filter, and run it. My friend's 1989 XJ has around 270k miles and has had a verified 7 psi idle for the last 5 years. It still runs great and gets 19 mpg highway. The 4.0L was known as one of the hardest engines to kill in the cash for clunkers program, where it kept on chugging even with liquid glass in the engine.

You should be fine.

 

[Redsnake]

Sounds to me like you should be fine (oil pressure wise) as others have stated. If you're getting 45psi at cold startup and it's staying above 10 pis hot idle, then just let your son drive it. (This is your son's first Jeep right???)

You took care of the main problem w/the 2001 XJs and replaced the Cylinder Head. Assuming that work was done correctly and you're no longer loosing coolant, etc... you should have a lot of life left in the '01 assuming other routine maintenance is done. The only other thing you might have issues with is a cracked/leaking radiator, but that's just another Jeep thing and not a hard or very expensive replacement.

I'm in the boat you were in before you replaced the Head on your '01... my cylinder head is likely cracked (or busted head gasket) so now I'm working to get it to the shop for some R&R and likely a replaced Head. Can you tell me (can PM me if you'd like) where you got your replacement Head and if the $2K was the price for that work or if that also included other stuff? Thanks.

 

[norm09]

I am having the same symptoms with my 01 the distributorless 4.0 has some design issues that causes the cracked head... The oil pressure problem is probably caused by worn bearings from coolant in the oil. The 4.0 is a great engine though ive been running mine with very low oil pressure for over a year... The symptoms are much worse in the warmer weather and by next summer ill be doing a rebuild. My advice for you would be run it try not to sit at warm idle alot and if the check gauges light comes on hold the throttle at about 1000 rpms this will raise the oil pressure enough to keep the engine safe. Its a very common issue on the late xj they make aftermarket heads that are built to handle the extra heat from having 2 cats under the hood... Sorry for the book and the bad news but a proper fix is the deefed up head and new crank rod and cam bearings.

 

[bryan joyce]

i have similar problems with my 2000 cherokee sport. i'm having a mechanic test the REAL oil pressure today with a mechanical guage. my oil pressure drops to almost zero when the engine is at max warmth and in idle. check engine light comes on. then, when i drive, it goes up to about 40psi. oil on dipstick looks normal. i don't appear to be losing oil or any other fluid. still worries me though, being that i'm taking it on a long roadtrip this summer; or maybe. it's got less than 100k miles on it. sure hope it's the sending unit or oil filter or something like that. it's awfully cold in colorado; wonder if it's just improper oil weight. any ideas?

 

[nwautoxer]

Suggestion: Use Mobil 1 10w-30 High Mileage oil.

Background: My 1998 XJ 4.0L with 5 speed transmission and 140,000 miles would drop to 0 psi at idle after running hard in the sand. Oil psi at freeway speeds was in the mid 30's. Changing to the 10w-30 Mobil 1 gives 10 psi at hot idle and almost 50 psi at freeway speeds.

Observation: Volume of oil being pumped is more important than pressure. If I rebuild the engine I will change to a high volume pump.

 

[GREASYHANDS]

Changing the oil to a thicker one slows down the flow. Less flow = less lubrication. Less lubrication = more wear on bearings. You dont need to have a high psi for good lubrication, the thinner the oil the faster it can get to the bearings and keep them cool and lubed.

 

[GREASYHANDS]

For the record I run 5w-30 in my 4.0 with 277k+ miles.

 

[svtkid]

My opinion, just drive it. The pressure does seem a little low but that shouldnt be a big problem. Also you said it was clunking. Is it the normal 4.0 clunking or is it louder and rpm dependant? Maybe check out your main/ rod bearings. A bad bearing could cause low pressure. P.S. I would avoid synthetic oils if I were you.

I run 10W-30 of whatever is on sale in my renix 4.0 with 240k on the clock and I have 65psi at idle on cold start and 25 psi when warm. Its always consistent no matter the outside temp, and it ALWAYS starts. Whether its 102F like in the summer or -2F like it was the other night. And mine clanks like a mother.

 

[cnjcherokee]

Throwin my pennies in
I have the same 2000 sport, and bought it the same way, oil pressure starts high and falls when it gets hot all the way down to 0, then follows the RPMs
The dealer had the sending unit and pump replaced and nothing changed.
The mechanic said it could break down tomorrow, or it could drive for 60K miles

 

[garr]

Throwin my pennies in
I have the same 2000 sport, and bought it the same way, oil pressure starts high and falls when it gets hot all the way down to 0, then follows the RPMs
The dealer had the sending unit and pump replaced and nothing changed.
The mechanic said it could break down tomorrow, or it could drive for 60K miles

What oil (Weight, Syn or dino) & What filter?
Many dealers & Oil change places use crap filters & whatever Bulk fill weight oil they have on hand,
5W20 seems to be the bulk fill oil choice of the day in most shops & it will result in near "0" hot oil pressure in most Jeep inline 6's, This happened once to me & I thought my Jeep was heading for an engine rebuild; I swapped in an M1 filter & M1 10W40 & my pressure went up to 13+ @ hot idle.

 

[Ecomike]

Volume of oil being pumped is more important than pressure.

I disagree with that. The physics of that statement is just wrong. Here is why. If I have a pump that is feeding two locations, and the pump pressure is 30 psi, then chances are that unless one of the two feed lines is blocked, I can assume oil is getting to both locations. If one of the feed lines is blocked, I may be in trouble even with 30 psi.

Now lets open the feed line (to simulate faster flow to one line due to a drop in viscosity, less back pressure due to less resistance to flow), to one of the 2 points of use, so much so that the pump line pressure is now essentially zero at the pump outlet. If there is any restriction difference between the two, back pressure on the second line, or if the flow is larger to the first line, it is possible to have negative pressure, (sucking) on the second line, it is called back flow!!!! When the engine rpm slows, and the pressure is dropping, it can also cause back flow from line 1 to line 2 as the pressure drops.

At best, at zero psi, you can expect the oil to take the path of least resistance, and not flow to places where it takes more pressure to get the oil to the spots where it is needed most. Imagine a pipe tree, with an outlet at an elevation of 1 foot, and another outlet at an elevation of 10 foot, then imagine you have the pump running at 15 foot of head at the pump outlet, gauge pressure. Now you throttle the pump to where the pump outlet gauge reads only 2 foot of gauge head pressure. At that point the flow of water out of the top outlet 10 foot of elevation will be ZERO!!!

Also you are assuming that the oil pump is pumping (actually moving) the same volume of oil at 200 F that it is pumping at say 100 F. But if the pump gear teeth are worn, it may actually be pumping less oil at 200 F than at 100 F, with the pump at equal speeds (same idle speed) as it is easier for the oil to slip and leak backwards through the gear teeth when its viscosity drops too much!!! Many of today's MV oils use polymers to simulate the high mol weight at operating temps, but these polymers have a bad habit of shearing and letting the oil viscosity drop on long drives at high temps and higher rpms. Some formulas seem to hold better than others!!!! When the oils cool off (like for an hour or more) they seem to recover from the high shear.

I notice the high shear effect when driving at 60 mphs for 30 minutes or more, but do not notice in stop and go driving for an hour!!!!!!

 

[Ecomike]

Changing the oil to a thicker one slows down the flow. Less flow = less lubrication. Less lubrication = more wear on bearings. You dont need to have a high psi for good lubrication, the thinner the oil the faster it can get to the bearings and keep them cool and lubed.

That is not the whole story and not entirely true!!! A thicker oil will stay in the lube spot longer!!!

Less flow means less cooling to the bear surface, but does not mean more friction. A thinner, runnier oil can me more friction, less lubrication.

You need flow and and FILM STRENGTH both!!!!

Viscosity is some what related to film strength.

 

[Ecomike]

This is an old topic to me, but a new idea is what if the 80 psi excess oil pressure relief valves are stuck open just a little, enough to bleed the low flow rate oil flow at idle, but not noticeable at higher flows?

Where is the pressure relief valve on 4.0s anyway? In the oil filter, or oil filter adapter, or in the oil pump assy, and the oil filter?