Well whoopdeedoo, another issue with my wife's XJ. A couple weeks ago I replaced the rear main seal, oil pan gasket, and valve cover gasket. When I dropped the oil pan, the pickup screen was clear, and I found almost no sludge or debris in the pan. So it's got new Mobil1 and filter. After the work was done, it ran great, solid oil pressure and no leaks for two weeks under normal use.

Well now, it starts up cold and has normal oil pressure. As it warms up, you can watch the needle drop slowly. After it reaches a steady state ~200-210 degrees, it drops down to zero and the "check gauges" light comes on. It's got plenty of oil and the pressure rises as the RPMs do, but still very low and drops down to zero at idle.

I'm planning to replace the oil pressure sending unit, but Google searches and forums have me freaking out that it's not going to be that simple and may be bearing issues or one of those phantom problems that is impossible to track down.

Any ideas or experience with these symptoms would be appreciated. Thanks!

2/8/2011 11:02:30 AM

replace the sending unit & report back with the results

2/8/2011 11:19:23 AM

My xj hated mobile one synth and spun a rod bearing about a week after i decided i'd be nice and give it a synthetic oil after years of dino.

2/8/2011 11:21:06 AM

^Yeah, not the first time I've heard that switching to syn caused big problems. I should've mentioned that I've been running Mobil1 in it for the past 80k miles.

2/8/2011 11:31:35 AM

Sending unit
Oil pump
Rebuild engine

I'd say it's probably the crank bearings. It sounds like the sending unit/oil pump is working fine if the pressure doesn't drop until the oil warms up and thins out. Not sure of any way to test a sending unit or oil pump other than to swap them, but I'd think you'd want new ones on a rebuilt engine anyway. You could try running some oil thickener, but even if it works you're just buying a very short amount of time before a full rebuild anyway.

[Edited on February 8, 2011 at 12:13 PM. Reason : s]

2/8/2011 12:11:57 PM

Yeah I'm pretty sure "sludge" replaced my bearings and the new oil cleaned it out

Ohh well.

2/8/2011 12:29:31 PM

what I don't get is how it happened so suddenly. After doing all that work a couple weeks ago, I've been watching the oil pressure gauge like a hawk, checking for leaks, etc. and the pressure has been holding great.

Am I wrong to think that if bearing clearances were getting bigger that it would be a steady decline over a period of time and not something that seemingly happened overnight?

I'm outside of my comfort zone here, so forgive my ignorance. I appreciate the insight and advice.

2/8/2011 1:50:45 PM

Happened overnight on my buddy's 351M powered boat. He blew a head gasket earlier that year and drove it like he stole it; so those two things probably exacerbated the problem though.

[Edited on February 8, 2011 at 1:55 PM. Reason : s]

2/8/2011 1:54:32 PM

When my bearing spun it did the opposite it blocked the oil passage and oil pressure went through the roof.

2/8/2011 4:05:01 PM

The only way to really check this problem is to get a mechanical test gauge and screw it in where the oil pressure sender goes. Crank and run until warm. Observe pressure.

BTW, you can get a standard 1/4" NPT hydraulic pressure gauge at Northern for dirt cheap. A little 2" or 2.5" one will do nicely.

2/8/2011 4:10:24 PM

^Thanks for the heads up on the cheap gauge. I knew that was the best/only way to determine if it was a lack of pressure issue or an erroneous sensor. The gauge(s) were as much or more than the sensor, so I figured I'd swap it out. Good call, and thanks again.

Now just cross your fingers for me!

2/8/2011 4:35:02 PM

Sounds like a pretty typical Jeep problem. Two of ours have had problems with displaying oil pressure correctly. However, pretty sure both of our issues have been a sending unit as our indicators have been slightly different than yours.
Good luck finding the source of your problem. Be sure to post back any progress!!

^^ We don't have a Northern Tool down here - think Tractor Supply might have it? I wouldn't trust a gauge from Harbor Freight... too many have had obvious issues. I'd love to run this test on my Grand. Thanks, Dan!

[Edited on February 8, 2011 at 7:01 PM. Reason : ]

2/8/2011 7:00:02 PM

Well, excellent news so far. It seems like the sending unit for now. I replaced it, let it warm up in the driveway with success, then took it for a ~5 mile test drive, then let it sit in the driveway some more. No drop off below 20 and no red freak out light.

Cold crank oil pressure held at about 45-50, then fell to ~40 when the temp rose to 200. I was pretty satisfied at that point but went for a test drive anyway. Normal rolling cruise at about 1500 rpm was reading ~45psi, rose to 55-60 at higher rpm. At idle at a stoplight, pressure fell to 25-30, but that's at below 1000rpm so I'm OK with that.

Honestly, it was easier to just replace the sensor since I dont have a convenient NT or HF around. I'll definitely get one if the problem rears its ugly head again.

Thanks again y'all.

2/8/2011 9:48:39 PM