Went on a day trip in the jeep and noticed my oil pressure lower than normal. I was pushing it up hills and when I was coasting I looked down and noticed it low. On cold startup, I sit just above 40 and it rises with RPM increase. When it warms up, it gets down to about the 1/4 mark on the gauge, the white line or just below. It will still rise with RPMs, but when it is idling it'll just stay down there. Is this too low? I'm not sure what the spec is supposed to be on these things. I ordered a new sender since they're cheap and easy to replace, but is a new oil pump in order?
Not that this is of any help, but I'm having the same issue with my 1998 4.0L as well. Immediate thoughts are maybe oil pump. But I have no idea honestly, bought my jeep a month ago.
Low oil pressure at idle has always been a problem with the 4.0L engine in jeeps. I believe it has to do with the flow restrictions caused by the banjo bolt in the filter adapter. The reason I think this is because the 99-06 clean-side block has it's filter mounted directly into the side of the block without an adapter and doesn't have this problem. XJ's didn't use this block before they was stopped being built, but did get the 0331 head and coil pack in 2000. My 2000 WJ does have this block and up until I pulled it from service at 253K miles, never showed less than 40 psi at hot idle. The reason the engine was pulled was low compression across the board and a light rod knock in cylinder #5. Come to find out most of the compression loss was burnt exhaust valves. Much of that I blame on the gearing and lock-up converter. But that's another discussion.
Another problem is the oil pump design. It's an ordinary gear pump that uses 2 spur gears meshing together and carrying oil around the outside of the case between the gear teeth. A much better design is the gerotor pump that has a gear inside a ring gear and is used on most modern cars. Actually this was used in the 57 Ford cars so it's not something new either. The whole point is a gear pump tends to wear loose much quicker than a gerotor pump does and should be replaced sooner. The rumor I've heard is around 125K miles is best. A high volume pump just has thicker (taller) gears in it but requires a deeper oil pan or modified pickup. Some say it just needs a modified baffle. It may improve pressure at idle because the volume is higher pushing more oil thru that stupid banjo bolt. I really don't know. The pressure is controlled by the pressure relief valve on the pump itself. I think it's designed to vent pressure above 90 psi, but don't quote me on that.
I've also heard that you can eliminate the adapter and screw the filter directly into the block on the XJ if you can get enough frame clearance to do it, but AFAIK, nobody has accomplished it yet. You'd have to find the right filter or bump out the frame. I'd like to experiment with modifying a banjo bolt if I had the time, but there's a fine line between drilling it out larger and making it snap under pressure.
To be honest, I can't say. I noticed it just the other day. I haven't made a habit of looking down at the oil pressure, sadly. I can't speak to how long it's been happening. I noticed it after the oil change, I suppose. But, I don't have any substantial evidence other than that's when I happened to notice, ya know?
After driving S***boxes most if my life, I've learned to keep an eye on the oil pressure constantly. The first thing I've done is put gauges in cars that only have idiot lights. The best thing about Chrysler cars is a full set of gauges. AMC jeeps didn't.
After driving S***boxes most if my life, I've learned to keep an eye on the oil pressure constantly. The first thing I've done is put gauges in cars that only have idiot lights.
I periodically look down and check, but it's not something I look at as much as temp. That's a fault on my end, and I agree it should be monitored more constantly.
