Got a fun new problem that I don't have the experience to diagnose... was driving on the interstate the other day and noticed my oil pressure randomly shot up to 80 then down to 0 by the time I was able to pull over. Got it towed home, replaced the oil pressure sending unit, drove it around and it seemed fixed until it got up to temp, now at temp when I idle my pressure drops to 0. I'm currently not driving it at all. I'd like to know what troubleshooting steps I could take to see if this is a cheaper problem than a full engine rebuild... I've seen around that cheap oil filters can cause this issue, but I've been using the same Mobil One filters for years without this issue. I don't have the tools or expertise to do a rebuild, or to even drop the oil pan as I'm pretty sure that requires removing the front axle. Are there ANY other inexpensive parts that could be the issue?

For some background, when I bought this thing it had the dreaded cracked 0331 cylinder head (which I wasn't aware of), which has been replaced with an aftermarket reinforced head from Clearwater. When the head was cracked I only drove it for maybe a handful of miles... was that enough to destroy the bearings? Also, that was back in 2015 or 2016... would it take that long for this problem to present?

 

You MUST verify your oil pressure. No guesswork for this and it is easy to do. Call around to local parts stores and find who can rent you a "mechanical oil pressure gauge"; will be inexpensive. Remove your oil pressure sending unit, install that gauge in there and take oil pressure readings. This way, you will know for sure. Post your results here for comment.

Also, there can be a constriction where the sending unit installs, try ramming a pipe cleaner into the ports to make sure that the gauge is reading oil pressure properly. Also, it is possible that your dash oil pressure gauge is faulty. Less likely but certainly in play here. If you are truly at zero oil pressure, your engine will definitely be making a lot of noise so if no noise and zero oil pressure on your gauge, what you are seeing on the gauge likely isn't accurate. Is your engine making noise??

FYI, the oil pressure spec for the 4.0 is as follows:

*Not less than 13 psi at HOT idle

*Between 37-75 psi OVER 1600 rpms.

Good luck and keep us updated!

 

Yeah that's a good call. It'll have to wait til tomorrow but I'll post my results when I test it.

 

Change oil/filter .

 

I would try a new oil filter first. It could just be defective. Easy and cheap...lol

 

If it is the pump you don't have to remove the axle , Just raise the Jeep as until the tires are airborne and drop the pan and remove the 2 oil pump bolts the pan will come out pretty easy but if you just change the pump there isn't a real need to remove the pan. Just install the new pump pack it with some grease or Vaseline . Or prime it with oil . You can do that with a tool or take out the spark plugs and crank the engine until you see pressure on the gauge.

 

Sorry that it took so long to get back to this, but finally bought a gauge and it's reading pretty low at temp, also noticed a ticking as it was idling (lifters?). I'll go ahead and change the filter/oil again, but other than that what would the next step be in diagnosing?

 

New oil/filter didn't do the trick

 

What oil did you use?

 

It had Mobil 1 10w-30 high mileage full synthetic in it which is also what I changed it out with. Switched out the mobil 1 filter for a Mopar equivalent this time

 

Just to get an elephant out of the room... any leaks anywhere that would cause a low pressure reading like that?
as far as the previous 0331 issue... did it turn to a complete milkshake before it was replaced? I would say yes if that's the case, because it could clog some of the oil passages to properly lubricate things like bearings, etc.

 

Yeah I noticed milkiness on the oil cap back when all that happened, I didn't drive it and they claimed they did a whole seafoam treatment to clean out the cylinders while the head was off.

I have noticed a very slow drip/leak that I haven't been able to track down. It's slow enough that the oil level doesn't drop noticeably between oil changes and I haven't noticed it get any worse, though it was parked at an airport for a couple weeks in subzero temps right before all this happened.

 

A little seepage as a vehicle ages is very common. A small one that doesn't make you have to add oil between changes isn't that big of a deal usually.
Compression test maybe? but a motor would have to be pretty shot to do that... how many miles on it? Do you have that milkiness now too? I'd try to track down the leak to either rule it in or out for being part of the problem or not.
Also I would run it by Cruiser here and see what he thinks on it. He knows a lot about our old 4.0s.

 

A lot of people, including myself, recommend using Rotalla T6 5W-40 oil. The higher viscosity will result in a slightly higher oil pressure.

 

No milkiness now that I've noticed, got an aftermarket reinforced cylinder head from Clearwater along with gasket and whatnot, and it's been like 3 years since that happened I think. Leak might be coming from the back of my valve cover, but I've replaced the gasket and tightened down the bolts as much as I feel comfortable doing without snapping them.

Was considering heavier weight oil, but will it really be enough to bring pressure from 3-5psi to over 13?