markhoff1022

I purchased a 2000 Jeep Cherokee Sport back in December. Inspections, test drive, and the Carfax didn't show that the past owner blew the head gasket. I took it to a shop to replace it, among a few other things.

Long story short, I have low oil pressure at idle after driving around for a while (the dial occasionally goes to zero and the "check gauges" light comes on). It's worse with the warmer weather, which is also a reason it didn't show up in the winter up north where I bought it.

I am a college student in Alabama and will have an internship in Montgomery, where I have to drive it every day, most likely in rush hour, where it's 97 on average in July. It still runs great- no rough idle, 35-40 psi at around 2,000 rpms, no check engine light or anything. I just replaced the oil pressure sending unit and no luck.

Some people say using 15w-40 oil instead of the recommended 10w-30 can make it last a few years longer (no clue how many miles that is). Is that my only option? I really don't want to sell it and am open to engine swapping, but I have no idea which shop to choose from- after calling around, I've learned that not many places do it. Of course, cost is a huge issue as well.

What are my options? Anything wrong with using thicker oil if I just need to buy some time until I make real paychecks so I can swap it? I recognize I'll have to swap it eventually.

GreaseMonkey17

So it HAD a blown head gasket but you had it repaired and it also has low oil pressure? Or you think that it has low oil pressure because of a head gasket ? Confused by your post .

Dave51

I am not a big fan of going the heavier oil route. It is not "oil pressure" that lubricates the engine, it is "oil". Going to a heavier weight may give you a better number but reduce flow to the more distal areas, depending on what is wrong.

So more important(ly) you need to figure out why it's low. Do you have any engine noise like rods or lifters? Have you changed your oil filter 5 times (cause elves are running around filling up oil filters with epoxy and putting them back on the shelves at WalMart)? Did you measure the oil pressure with a mechanical gauge? Did you purge the oil sensor passage prior to installing the new sensor? Did you check the wiring to the sensor? Did you pull the oil filter adapter and see if that bolt was occluded? Did you pull the pan and check the oil pump, screen and pressure relief? If you have a bearing problem and they're only a little worn, a better band-aid (or even permanent fix) might be going to a high volume oil pump.

00t444e

I use 15w40 in mine and it increased the oil pressure by 5-10 psi, and it runs quieter. Also make sure you are using a good oil filter like Motorcaft, Mopar, Wix, or Purolator. The 4.0 engine can use as high as a 20w50 oil, they only spec 10w30 because that is the lightest weight you should use, and in theory you will get better fuel mileage with lighter weight oil, but switching to 15w40 made no noticeable difference in fuel mileage for me.

99XJ93YJ

Where did you get the oil pressure sending unit? did you use a torque value? I replaced mine twice as the first one was either broken or I possibly didn't torque it correctly, either way, my oil pressure would be fine then dip to zero and when that happens the check gauges light came on. It was really frustrating.

dave1123

In February of 2018, I bought a 2000 Cherokee Sport with 126K on it. It had 30 psi oil pressure on cold start and it went down to 10 psi after it warmed up. It got up to 30 while driving, but never any higher. IDK what oil it had in it, but I replaced the oil pressure sensor with a proper Mopar one ($70). At that time I found out the one for the 2.5L was a different number. I switched to Rotella T6 5W-40 synthetic diesel oil at the same time. Now my cold startup pressure is 45 psi, hot idle is 30, and it goes up to 50 at anything over 1500 rpm. It starts normally at -15*F as well. The oil is a bit pricey at $30/gal. but I buy it at Advance Auto when it goes on sale for $19.99/gal. which is usually once every couple of months. I'm using 5K change intervals and it barely changes color in that time. I'm also using Purolator One filters.

In following the advise of Cruiser54 on the oil, he was right and I'm not going back to anything else except for an emergency top-off.

BTW, the first rule of hydraulics states that "pressure is equal throughout the system." therefore unless restricted, flow will be equal as well. If your gauge is showing 40 psi, it should be 40 psi at the bearings as well. Having said that, I think the gauge on the Cherokee is before the filter in the flow path. The gauge on my WJ is in the lifter gallery. She's got 250K on her and she's still showing 40 psi at hot idle.

markhoff1022

The shop said there was a ton of sludge on the cylinder head, either from missed oil changes or because the coolant made the oil solidify. Either way, I just put in 6 ounces of Seafoam into the oil to hopefully break up an sludge that might be restricting any oil flow. Best case scenario the oil pump screen is clogged and this will clear it up after a few hundred miles or so of driving. I have a Mopar filter waiting to go on so I'll flush it out and put in new oil (still undecided on the weight) and I'll see if there's any change.

I'll see if I can mechanically test it some time soon. I'll admit I'm not too knowledgeable with cars (thought I did get an XJ to start learning) so when I test it I want to make sure someone else is with me to help give an accurate diagnosis. What's the big benefit of the mechanical test? I would have thought a new sensor would be quite accurate.

Dave51

Actually, that's right up there with things in the worst case scenario because you're driving around with little to no oil pressure for a few hundred miles or so.

Do you have rod or lifter noise? If not, then it is unlikely that you truly have zero oil pressure, but given the risk-benefit of what we're talking about here it is critical that the cause be determined quickly. To answer your question, you could have an out-of-box sensor failure; if you bought a Fred's Bargain Oil Sensor it could be inaccurate; or you could have a wiring issue. In the water under the bridge department if you had tested the pressure mechanically beforehand you could have saved the cost of a new sensor (which may not have been a big deal if you bought a Fred's Bargain Oil Sensor).

Yeah, that was kinda key information. IIWY I'd purge that oil sending unit passage. The new sensor could be clogged now too.

Dave51

This is incorrect. Pressure is equal throughout a system only when the system is closed, such as the brake circuit. This is Pascal's Law. In an open system we need to apply Poiseuille's Law and calculate pressure drop, adding in the complicating factors of turbulence and increased viscosity. Further, since there is a pressure relief valve in the oil pump, additional flow may be lost.

In re: the location of the oil pressure sensor, it is

Dave51

Gotta be careful talking about oil here. People get thrown off the board!

Alabama in the summer huh? I go to 10W-40 when it's hot. I just don't like 15W or 20W.

If your engine is a little worn but no parts in immediate danger of failing, a better option is to provide more oil by going to a HV pump. Melling makes them, and there is a Mopar product:

Mopar P4529241 High Volume Oil Pump

dave1123

Yes, I had to get off my *** and go out and check and the oil pressure sensor IS after the filter. AND you've got a pressure drop thru the restriction of the banjo bolt. I think you would see a significantly higher idle pressure if there was enough room to screw the filter directly into the block. Thank you for refreshing my hydraulic knowledge as it's been over 20 years since I've worked in hydraulics. Upon inspection, it's not just that gauge sensor elbow that might plug up with sludge, but the small passage that's drilled into the filter discharge passage as well. Take the elbow out and clean it AND probe the passage into the block. As has been stated, if the bearings aren't rattling, there must be sufficient pressure to support them.

IDEA. You could use a drill bit in a hand chuck to clean the oil passage into the block and the drill flutes would lift out the gunk.

About Sludge. It's formed by water not being evaporated by engine heat, co-mingled with combustion by-products such as carbon and airborne dirt. If there's no leakage from the cooling system into the engine, sludge is formed by not allowing the engine to run at operating temp long enough to evaporate the water that enters the engine from the air when it cools down. Short trips are usually the cause of this, especially in the winter when the air is very damp. SAE recommends at least 14 miles or a half hour after reaching operating temp for this to not occur. The engine must operate at at least 180*F for the water to evaporate. Sludge first appears as a white glue-like substance, then turns brown or black as it picks up deposits. When it reaches high heat, it can bake into extremely hard deposits that require heavy pressure to chip away.

KJamesJR

If you're going to flush your oil, use a cheap conventional brand. It's going to become contaminated rather quickly and you're going to need to flush it again within a couple weeks until you can get back to normal intervals.

I'm kind of confused about this thread because you said it didn't have any issue but you still got the head gasket replaced? Was your mechanic sure it was a head gasket? Was there coolant in the oil? Does your new oil, with the new head gasket still look milky/coffee cream colored? Do you have any massive oil leaks that could be interfering with your pressure readings? There's a lot more you can check before you start trying fixes that may or, most likely wont work. You may want to return your oil pressure sensor and pick a mopar part up from a dealer.

dave1123

It's times like this that I realize how difficult it is to diagnose a problem with a computer keyboard! You have to visualize in your mind what you're working on.

Dave51

See previous thread:

https://www.cherokeeforum.com/f2/adv...ngines-250024/

Frankly, I can't see where it's ever been determined if that thing still has a 0331 head on it or not. But with only 107,000 miles on it (give or take) I'd really make an attempt to save it.

dave1123

You are right. I'm assuming it still does have the 0331 head on it because of the "low" mileage. Problem is even the TUPY is an 0331 but didn't come out until 2002.