So I started my 99 XJ this morning, after a decent rain yesterday, and about 25F this morning when I left, and the oil pressure gauge was pegged at 80psi. I first think well crap a wire/ plug broke, but then I notice the pressure drops to about 55-60 psi at idle, so now I am thinking this makes no sense. So then when I shut it off I turn the key back on, and the needle jumps straight to 40 psi with the jeep off. Normally its at 0 psi with the jeep off. So some how it seems the entire gauge has been shifted about 40psi, but why, and where is the failure? Something mechanical? I really don't think so, there is no noise, smoke, smell, leaks, or anything. Something in the sender unit? I think probably the most likely, but I am not sure. Something in the gauge itself? I really don't think so, but maybe. What are your thoughts? Hopefully I can poke around under the hood tomorrow.

Sean

Since it's the sender that provides a variable resistance based on a mechanical pressure, that's where I'd start. For whatever reason maybe the resistor part is somehow shorting or became more resistive, depending on which way those things work. That's my entirely unprofessional first hunch anywho.

Times two!! And Sean, one thing we do know...it ain't your actual pressure!
Big JIm <img src="/forums/images/graemlins/pfft.gif" alt="" />

It was the sending unit. I unplugged the sensor an turned the key on and the gauge stayed at zero, started up the jeep and the gauge didn't move as expected, and the check gauges light came on as expected, so I ordered a new unit Tuesday evening (Napa didn't have it in stock) picked it up Wednesday morning, put it in, and the jeep is good to go. It was a pretty easy swap too.
<img src="/forums/images/graemlins/cheers.gif" alt="" />
Sean

so interesting observation. With the old oil sending unit before it failed, I was running around 30 psi at idle and max out around 60 psi. Now with the new sending unit in there, I idle roughly 37 psi and max out around 52 psi. The increasing range of oil pressure could be an indicator that a gauge sending unit is going bad, maybe not, but just thought I would throw it out there.

Sean

What? Looks like the range of PSI has gone down. From 30-60 to 37-52. What's wrong with that? Sounds like it's doing its job a bit better than the last unit before it went south entirely. I don't pay them no nevermind nohow. They never do seem to work quite right. If an idiot light comes on - STOP. But you know that....

Well the plastic part of the sensor was no longer potted inside the brass portion, so it didn't surprise me the sensor was not reading right when I found this, and it would seem to reason that after 140K miles the accuracy or the band of the sensor would change or fall out of spec. I know the actual oil pressure didn't really change, but rather the just reading the sending unit was sending is all that changed, so nothing is ôwrongö anymore. And personally, I would rather have gauges for everything over idiot lights, a gauge will at least allow one to trend data so you have a reference of whether or not something is different, I mean ultimately I trust my eyes, ears, nose, and mechanical knowledge, but gauges are nice to have for reference. The oil pressure gauge is typically the first thing I look at when I start my Jeep too, if I donÆt have oil pressure there is no point in movingà.

Sean

Educate me a bit on oil pressure readings. It's my understanding that if there is oil in the engine, even if at a dangerously low level, the pressure will read correctly (assuming the sender is working right). Is that correct? If so, what is the gage going to tell you that the idiot light won't? Maybe that if the pressure is trending up or down over time that something else is going wrong? Every time I've seen that it's been the sender itself though.

There is no way to trend a failed light bulb.

When the oil gets dangerously low the pickup will suck air from time to time and the pressure will vary from air to oil
with the lifters protesting when they get a gulp of that air. Also the lower the oil level is the HOTTER the oil will get as it is reused more often and doesn't have a chance to lose it's temperature before reentering the engine.

The lights were tried many times in the past. Buyers of the vehicles proved to be more conscious of the readings than the lights would provide.. So back to the gauges we went. Most modern gauges are simply glorified lights! When 12 pounds or so are reached the needle goes to the MIDDLE of the gauge. Then when the pressure drops to 12 pounds or less the needle goes to the bottom..
For most engines 20 pounds of oil pressure will work as well as 40 pounds under all but the most demanding of circumstances.
So that said, the manufactures of the senders and gauges are rather reckless in having EXACT measurements. Although your TWO senders have sent different signals to the gauge, I'm sure the pressure hasn't changed and doubt if either are CORRECT!.
For my own use I'd just as soon have a light...and even with a bell would be Ok too.
Hell! My new pickup even tells me when I'm a quart low.. It hasn't told me yet but someday it will.
Big Jim <img src="/forums/images/graemlins/pfft.gif" alt="" />