Dipping oil pressure?

[Dyne]

Hey everyone, I am new and this is my first post. I joined a few weeks ago and have been lurking for a month or two. I just sold a 61 chevy SWB and full box chevy frame to start working on my 98 XJ. I'll probably be waiting a little while to start a build thread since I haven't even started on it yet. Anyway onto my question.

I was driving up to Vancouver today from Portland to help a friend with some computer trouble. Got off the freeway and stopped at a light, looked down at my oil pressure gauge and it was at 10 psi, successfully freaked me the **** out and i was about to turn the motor off, but it stayed at 10. Now here's the thing, it goes up to 40 under light acceleration and hovers between 30 and 40 at speed (35-40) its at about 45 at 60mph. Is this the pump or the send unit? Engine wasn't making any odd noises at 10 psi, my oil level isn't low. No oil leaks (other then the filter elbow, which is next to nothing) but i am pretty due for an oil change.

[Rayburne]

This is scary as my 97 XJ started doing the exact same thing about a week ago when I was climbing a gulch out here in the desert, it wasn't hot out and the climb was difficult and my rpm wasn't high either. I thought I had punchered the oil pan so I got out and checked for oil on the ground and checked the oil level but everything was fine. I'm thinking of changing my sending unit and also adding a mechanical gauge. Maybe someone else has had this problem and knows the fix.

[Cheromaniac]

Got off the freeway and stopped at a light, looked down at my oil pressure gauge and it was at 10 psi, successfully freaked me the **** out and i was about to turn the motor off, but it stayed at 10. Now here's the thing, it goes up to 40 under light acceleration and hovers between 30 and 40 at speed (35-40) its at about 45 at 60mph. Is this the pump or the send unit?

It does sound like the oil pump pressure relief valve spring could be weakening but I'd replace the sending unit first and if the oil presure readings are still the same, pull the oil pan and replace the pump.
Both are cheap fixes so I suggest you do them ASAP before any engine damage occurs.

[NW99XJ]

I'm going to lay odds it the oil pressure sending unit.
On your late model, its right above the oil filter.
I live in the PDX area too, I'd be happy to look at it for you if you wanna hook up at some point this weekend.
Otherwise, check out the plug that connects the sending unit to the system. Its a three pin plug I think.
There may be a sliding release clip on that, that you'll have to disengage in order to separate it. If there is any oil at all in there, then your switch is bad.
It'll look like this.....
http://i1104.photobucket.com/albums/h328/jbchill1/IMG_2297_zpsfe31c024.jpg
If not, I'm sure there is a test you can do using a DMM, but I don't know the procedure off the top of my head.
(the red part on the switch there is the sliding lock that you'd have to disengage.)

[Firemanray]

All of the posts above are certainly things to check if your gauge is acting crazy. If you truly do only have 10 psi at idle it's nothing to panic over. Rule of thumb is 10 psi per 1000 rpm of engine speed. If the pressure climbs as the rpm's do everything should be fine. One thing I have learned also is if you see a slight pressure increase on deceleration followed by pressure decrease with slight acceleration you could have worn main bearings.
Of course all of this hinges on the fact that the gauge must be functioning correctly to begin with.

[Dyne]

thanks guys, so pretty much what i figured i should look at. though i didnt know the 10 psi per 1k rpm thing. is there a reason for that or is it just the way it works?



I live in the PDX area too, I'd be happy to look at it for you if you wanna hook up at some point this weekend.

i'd be up for that, wouldnt have time till sunday i think. tomorrow i'm going over to my uncle's house, he's the one that's going to be helping me with my jeep as i have next to no tools currently. my truck was sitting at his house so he has the money from it so i'll be picking that up, as well as doing a walk around of the jeep, taking pics and deciding on the order of upgrades. i am going to be bringing up the oil pressure thing with him when i see him though. and after my uncle's place is wife time which'll take up the rest of the day. go ahead and send me a PM with what time and day would be good for you.

[Firemanray]

.......... i didnt know the 10 psi per 1k rpm thing. is there a reason for that or is it just the way it works?

Just a rule of thumb. No special magic about it.