New to the Jeep world so looking to get educated. My daughter is now the owner of an 01 Grand Cherokee with the 4.0L. Couple issues with it that I wanted to bounce off the group. First is the oil pressure seems to be all over the board on it. When going down the road the pressure is around 40, when slowing and stopping it will drop to about 10. Is there a for sure way to determine if oil pump is getting weak or if it is just the sending unit?

Screw a mechanical gauge into the pressure sensor fitting to verify the low pressure. There are several reasons for low pressure, the worst of which is worn bearings. It could also be the pressure relief valve in the pump. Are the valves clattering when you show 10 psi? If not I would suspect the sending unit (oil pressure sensor). If it's leaking, replace it. You'll find oil in the wiring connector or running down the side of the engine. On the 4.0, the sensor is susceptible to being whacked with an oil filter wrench, This is quite common.

A less common problem would be contamination by fuel. That you would smell on the dipstick. That is usually caused by a leaking injector, although you would tell by poor idle and a really hot convertor.

I've tried to cover all the possibilities so you know where to look. I'm not trying to scare you. It's probably just a bad sensor.

I have a weird problem on my 2000 Grand. Sometimes on cold startup, the oil pressure gauge pegs at the top. If I shut it off and wait about 2 minutes, then restart, it's fine. I haven't changed the sensor yet.

10 psi is within specs BTW, along with what was mentioned above there is a known concern with sludge/debris clogging the oil passage the sender screws into. The quickest, easiest, but messy way is to remove sender, start engine for 3-5 seconds. Keep in mind oil will spray out of the whole along with clog so be prepared for the mess. The other option is taking something like pipe cleaners, thin rag around a coat hanger and clean the passage out.

Another possibilty though its uncommon is a faulty oil filter. Depending on how it bypasses oil and how well the bypass works, an oil filter can restrict flow and cause low pressure. This is the part where some people dont like using cheap filters and it seems the leaf type spring design can be more problematic.

ive read that if you have 1psi of oil pressure for every 100rpm the engine is turning, you're in good shape.

10psi for every 1000 rpm is the general rule of thumb. Same as 1psi for every 100 rpm.

So by the sounds of the last few posts, what I am seeing seems normal?

I drove my company's Chevy delivery pickup for a year with 10 psi idle pressure. As soon as you raised the engine rpm, the oil pressure went up. I still was afraid it would die on me on the job, but it never did. It didn't rattle at idle either.

Google this and you will find MANY 2000 & 2001 4.0 with oil pressure problems. I have been driving mine for a long time with only 4psi at idle. Pumps,bearings are usually just wishful thinking and replacing any of these often does not alleviate the problem. My advice is if it isn't knocking and that is the only problem your having....just drive it.
There are too many misconceptions about oil pressure. Pressure is not a function of oil DELIVERY. It is a measure of restriction so it is useful to benchmark engine wear (less restriction=wear) However, it is not a measure of oil FLOW....what matters the most is how much oil is flowing to each bearing and lifter. Theoretically it only takes 1/2 psi of pressure to lift oil 1 foot ( the distance from the pump to the lifter galley) gravity will feed oil to everything below. Is this optimal? Of course not, but that is all the dynamics required to get oil from point A to point B. The Chevy sixes did not even have oil pumps until the early sixties.
Don't get me wrong,there is something causing low pressure but that does not necessarily mean your engine is about to grenade...so don't freak out.

If you put an oil pressure gauge on a fully roller bearing motorcycle engine, you'd only find 10 psi. It's FLOW that's important.

my 2001 GC has done this since I bought it a few years ago, it drops down to around 5-10 at idle and up to 40-50 under operation..... but sometimes at idle it stays at 40-50 so it's odd. I know it's a common thing but it's still hard for me to look at such an extremely low oil pressure reading so.......how can this be fixed for good? What does a guy have to do to keep a steady higher oil pressure at all times?

RIZ

I have heard too many stories like yours of 2000 & 2001 4.0s having low pressure (I am one of them) You can throw money at it if you like but there is NO guarantee it will raise your pressure. I drive mine all over the place in all types of driving,from stop and go to long distance interstates. When and if it starts knocking I'll put another engine in it.
IMO I would not lose sleep over it.