After researching everything I could find about extremely low oil pressure at hot idle with the XJ, my 2000 has just 135K on it and it holds 20-25 psi at idle hot (215*). As an experiment, I removed the adapter and screwed a Wix 51348 oil filter directly into the block using the filter nipple from the adapter. This filter is smaller than the 51085 that's supposed to be on it, but works just fine. If you're not happy with using a smaller filter, change it more often.

ANYHOW, the results are interesting. It will hold 40 psi at 195*, 35 at 210* and 30 at 215*, immediately rising to 40 at 1000 rpm and 50 above that. I'm using Rotella T6 5w-40 oil. This filter clears the frame by 1.125" and the motor mount by .625" (5/8). It is well below the mount's thru bolt and unless the trans mount breaks and the engine shifts forward more than 5/8", it shouldn't be a problem. The engine torques down on that side anyway. Time will tell if it works the way I expect it will.

The only problem I can see is now I have to locate my small filter wrench! It's OD is 2.921". The 51085 is 3.660".

 

Well the numbers certainly sound good so here's to hoping all goes well.

 

20-25 psi at idle isn't extremely low, it's well with spec, 13 psi is the minimum. I did consider deleting the adaptor on mine to prevent it from leaking, but I use the Motorcraft fl1a filter and there is no way that will fit without the adapter. I ended up replacing the o rings and keeping the adapter. I am using 15w40 diesel oil and have no problems with low oil pressure.

 

I'm an old gearhead and get nervous any time oil pressure gets below 40 psi. This is the first vehicle I've ever owned that had low pressure at idle and even though it's normal, it still makes me nervous. What's odd is my 97 ZJ never had low idle pressure and it's built the same way as my XJ.

 

Okay, I learned a few things about the oil filter adapter tonight. It DOES form a restriction, but only at high flow rates. Before I removed the adapter the only time I ever saw 50 psi was on cold startup. Now I'm seeing 55 on cold startup and it drops down quickly to 45 as the engine begins to warm. I still only have 25 psi when it's really hot at idle, but the gauge needle is a lot more active, bouncing between 30 and 50 as you drive around changing speeds constantly, mostly hanging around 45 when under 45 mph. It will hang on 50 at 65 mph though. If I flip the extended idle switch and it jumps to 1000 rpm, it will hold a steady 40 psi. I think the only way to raise that 25 at idle is to install an oil cooler. SO.... my opinion is to put the adapter back on and install an oil cooler.

OH.....and I had it wrong. The oil enters the filter first, then flows out thru the banjo bolt.