gvthnks Posted May 24, 2022 Share Posted May 24, 2022 Well, poop. Sometimes too much information is bad. Not really, I know, but I finished swapping out gauge clusters on my 87. Previously, when it was hot and idling, the oil light would flicker but would go out as soon as the throttle was raised and RPM increased. Now that I have a gauge, it tells a more complete story. Cold idle is about 40 psi, and it quickly drops to 20 at idle and when warm, all the way to zero. No knocking or rattling sounds, but the gauge all but bottoms out. Pressure still increases when I apply throttle and it runs about 35 psi on the gauge at 2k rpm. I know, I know. Gauges aren't always accurate and all that but it seems to tell the same story that the oil light did. This is the original engine and it has 171k on it. When I got this thing a little over a year ago, it was obvious that the engine hadn't been maintained well. Caked on oil in the lifter galley and cylinder head. I'll still continue to work on it and when ready I'll drive it and see how it does. I may drop the pan and look at the intake on the oil pickup to make sure it isn't clogged like some of the other passages were. I'm just expressing disappointment with it. I guess worst case scenario I could find a donor 4.0 or rebuild this one. Link to comment Share on other sites More sharing options...
Eagle Posted May 25, 2022 Share Posted May 25, 2022 Factory spec for warm (operating temperature) oil pressure is 13psi minimum at idle, and 37 to 75 psi above 1600 RPM. Typically, my experience has been that ***most*** healthy 4.0L Jeep engines idle at 20 to 25 psi and run 50 to 55 psi at 2000 RPM (highway cruise). Oil pressure as low as you are seeing probably indicates bearings, and those can be replaced with the engine in the vehicle. In fact, a number of years ago I hosted a meet-n-greet, BBQ, and trail ride for the NAXJA-NAC chapter. Folks showed up at my place on Saturday, we had a huge cookout Saturday night, everyone camped in the field in front of my house Saturday night, and we did a trail ride on Sunday. One of the guys dropped his oil pan and replaced main and rod bearings while we were cooking dinner. [Edit]Forgot to ask: What brand oil filter are you running? I'm not sure why, but Fram oil filters cause low pressure problems on Jeep engines. Also, what brand, type, and weight of oil are you running? Link to comment Share on other sites More sharing options...
Tactical Bacon Posted May 25, 2022 Share Posted May 25, 2022 There could be sludge inline with the OPSU causing a low reading. Your first step should be to verify oil pressure with a mechanical gauge. Next, it may be worth trying a crankcase cleaner, like dumping half a can of Seafoam in with the oil and driving as normal for a few days before you change the oil. I'd also suggest trying running a pipe cleaner down through the hole where the OPSU screws in, I had a problem with a port partially blocked by sludge on my 91 MJ and cleaning it out took care of chronic low oil pressure readings on my dash gauge. Link to comment Share on other sites More sharing options...
OldManComanche Posted May 25, 2022 Share Posted May 25, 2022 Verify you have a problem with a mechanical gauge. What weight of oil are you using? Link to comment Share on other sites More sharing options...
cruiser54 Posted May 25, 2022 Share Posted May 25, 2022 https://www.jeepforum.com/threads/4-0-low-oil-pressure-at-idle-fixed.1304679/ And, like Eagle, I'd like to know which filter you are using. Link to comment Share on other sites More sharing options...
Dickinson County Comanche Posted May 25, 2022 Share Posted May 25, 2022 20 hours ago, gvthnks said: Well, poop. Sometimes too much information is bad. Not really, I know, but I finished swapping out gauge clusters on my 87. Previously, when it was hot and idling, the oil light would flicker but would go out as soon as the throttle was raised and RPM increased. Now that I have a gauge, it tells a more complete story. Cold idle is about 40 psi, and it quickly drops to 20 at idle and when warm, all the way to zero. No knocking or rattling sounds, but the gauge all but bottoms out. Pressure still increases when I apply throttle and it runs about 35 psi on the gauge at 2k rpm. I know, I know. Gauges aren't always accurate and all that but it seems to tell the same story that the oil light did. This is the original engine and it has 171k on it. When I got this thing a little over a year ago, it was obvious that the engine hadn't been maintained well. Caked on oil in the lifter galley and cylinder head. I'll still continue to work on it and when ready I'll drive it and see how it does. I may drop the pan and look at the intake on the oil pickup to make sure it isn't clogged like some of the other passages were. I'm just expressing disappointment with it. I guess worst case scenario I could find a donor 4.0 or rebuild this one. My 89’ had a similar oil-pressure problem like this and it ended up being a worn out oil pump. I’d definitely check your sensors first though, just to be safe Link to comment Share on other sites More sharing options...
cruiser54 Posted May 25, 2022 Share Posted May 25, 2022 Take a peek into the oil filler with a flashlight and assess the sludge in there. Marvel Mystery Oil will melt that crap away nicely. Link to comment Share on other sites More sharing options...
gvthnks Posted May 25, 2022 Author Share Posted May 25, 2022 1 hour ago, cruiser54 said: Take a peek into the oil filler with a flashlight and assess the sludge in there. Marvel Mystery Oil will melt that crap away nicely. Thought about that as well. Link to comment Share on other sites More sharing options...
gvthnks Posted May 25, 2022 Author Share Posted May 25, 2022 2 hours ago, cruiser54 said: And, like Eagle, I'd like to know which filter you are using. I'm currently running an STP filter on it. Link to comment Share on other sites More sharing options...
cruiser54 Posted May 25, 2022 Share Posted May 25, 2022 Junk. Get a Napa Silver or Wix. Link to comment Share on other sites More sharing options...
Eagle Posted May 25, 2022 Share Posted May 25, 2022 8 hours ago, gvthnks said: I'm currently running an STP filter on it. What brand of oil, what series, and what weight? Link to comment Share on other sites More sharing options...
gvthnks Posted May 26, 2022 Author Share Posted May 26, 2022 15 hours ago, Eagle said: What brand of oil, what series, and what weight? Penzoil, 10w30 and it was dino oil. This oil has about 2 hours of idling time and 3 miles on it since it was fresh. I think this weekend, I'll follow the tips in the thread Cruiser shared and replace the oil and filter again after doing so. We'll see. Link to comment Share on other sites More sharing options...
cruiser54 Posted May 26, 2022 Share Posted May 26, 2022 Just do the filter if the oil is that new. Link to comment Share on other sites More sharing options...
Eagle Posted May 27, 2022 Share Posted May 27, 2022 1 hour ago, cruiser54 said: Just do the filter if the oil is that new. Agreed. 10W30 should produce adequate pressure, absent other issues. Link to comment Share on other sites More sharing options...
gvthnks Posted May 27, 2022 Author Share Posted May 27, 2022 14 hours ago, cruiser54 said: Just do the filter if the oil is that new. 12 hours ago, Eagle said: Agreed. 10W30 should produce adequate pressure, absent other issues. I'm going to run some MMO through it and dump it while it's hot. When I got this thing, it had been parked for about 11 years or so. I pulled the engine to replace all the expansion plugs and pulled the head to replace lifters while I was in there. There was gunked up oil everywhere. Some of it had to be scraped off it was so caked on. I have no doubt this thing wasn't maintained all that well at some point. So my intent was to get it running, run some seafoam /MMO / ATF in the oil to help clean it out and dump the oil pretty quickly. Life happens, and this thing has been sitting for a few months since I got it all running well again. Fast forward to now and I am working on it again. So that's why and where I'm at. I've had close to a dozen of these engines or maybe more over the years and believe it's a great platform. I've saved quite a few and am hoping this one still has some life left in it. thanks for all the tips and advice. I'll keep you posted. Link to comment Share on other sites More sharing options...
87MJJeep Posted June 2, 2022 Share Posted June 2, 2022 On 5/27/2022 at 8:40 AM, gvthnks said: I'm going to run some MMO through it and dump it while it's hot. When I got this thing, it had been parked for about 11 years or so. I pulled the engine to replace all the expansion plugs and pulled the head to replace lifters while I was in there. There was gunked up oil everywhere. Some of it had to be scraped off it was so caked on. I have no doubt this thing wasn't maintained all that well at some point. So my intent was to get it running, run some seafoam /MMO / ATF in the oil to help clean it out and dump the oil pretty quickly. Life happens, and this thing has been sitting for a few months since I got it all running well again. Fast forward to now and I am working on it again. So that's why and where I'm at. I've had close to a dozen of these engines or maybe more over the years and believe it's a great platform. I've saved quite a few and am hoping this one still has some life left in it. thanks for all the tips and advice. I'll keep you posted. I hope you have some positive news soon. Your username is clever and wise! Link to comment Share on other sites More sharing options...
Recommended Posts
Create an account or sign in to comment
You need to be a member in order to leave a comment
Create an account
Sign up for a new account in our community. It's easy!
Register a new accountSign in
Already have an account? Sign in here.
Sign In Now