So, new here, new (to me) 2000 XJ / 4.0 with 146,000 miles. Searched the many "low oil pressure" threads but my situation seems a little different.

When I bought the truck a few days ago, the oil gauge read zero and the "check gauges" light was on. As the engine ran smoothly and didn't seem to be trying to jump out of the hood, no valve tap or rattling noises, and the gage was zero and not low, I presumed (and still do) that the engine is more or less solid and the gauge sender is bad. The seller told me the original sender went bad, he installed a new one from Rock Auto, and it worked for awhile (not sure how long) and then went south again. Oil looks OK, looks like 2000 miles old (which it is according to the oil change receipt in the glovebox), level good. The instrument cluster diagnostic does what it's supposed to do, with the gauge moving through the range. The wire to the sender has 5V at the correct terminal.

Yesterday I installed a new sender (NAPA/Echlin) and now the gauge reads 10 psi... all the time, whether the engine's turning or not (as long as the key is in the run position; it goes to zero when turned off).

Today I pulled the sender and installed a temporary mechanical gauge. It read 5 psi at 500 rpm idle, 10 psi at 1000 rpm, etc., up to 30 psi at 3000 rpm, which is as high as I checked. A bit low but not alarming from what I've read here. Just to be sure, I tried another mechanical gauge and got the exact same results.

I took the sender back, but they don't have another one in stock (should have it tomorrow). Meanwhile, I'll change the oil (the PO had it done at one of those quickie places, with some generic cheapo filter). I'll see how it behaves with new oil and a quality NAPA/Wix filter, and hopefully a new sender will read correctly. But if it doesn't... is there any way to test the sender and/or the wiring? I have the factory service manual, and it doesn't say anything about that. The wiring diagrams just show that the sender is connected to the ECM, and the gauge is driven by the data bus, so I have no way of knowing what the sender should be putting out in terms of voltage or resistance or whatever.

Thoughts?

 

First, when you checked the oil pressure manually, did you do it with the engine hot? If you have an oil pressure issue, it may only show up when hot and not at cold idle.

Regardless, you're obviously getting a bogus reading at the gauge. Check the wires coming out of the oil pressure sender connector. My 2000 had all the insulation rotted right off those wires where oil leaking through the sender had eaten it right off. I had to cut open the wire loom to get up high enough into the wiring harness to find some good wire to splice a new connector into. Mine behaved a little differently from yours (grounded out and pegged the meter at 80 psi), but those wires are still where I'd start.

CCKen or Cruiser will probably come along shortly and tell you how to test it with a meter.

 

Yes, with it hot.

What's odd is that it read zero with one sender, and a constant 10 with another.

 

Auto parts store senders(even Napa) are pretty dodgy. The only one that works for sure is a Mopar one.

 

It's all good now. Took the sensor back to NAPA and exchanged it for another one, and also changed the oil. Now I have 20psi at idle according to the dash gauge, up to 40psi at 3000 rpm. Obviously a bad new gauge stuck at 10psi, and presumably (hopefully) just a bad oil filter.

 

I wonder if the first sender unit was for a light, not a gauge.

 

Same part number, identical appearance. I would think an idiot light sensor would be only one or two wires, not three as for the gauge sensor. But in this case I think either one uses the same sensor which connects to the ECM, which in turn sends a signal via the data bus to the instrument cluster, which interprets the signal and lights a light or moves a needle, as appropriate.

Sigh, the bimetal gauges in my CJ5 were a lot simpler. Unreliable, but easy to diagnose.

 

Well, not all good. Worked fine for over a week, now showing zero again when I left work for home. Tried the sensor that always reads 10psi running or not, yup, reads 10psi without even screwing it in, which makes me believe it's another bad sensor, not wiring. Guess I need to go to the dealer for a real Mopar sensor...

Oddly (and I think coincidentally) the check engine light came on briefly (10-20 seconds) during the same drive. Stored code when I got home was P1495, light is still off. We'll see what tomorrow brings.

 

You might want to check the connector, those go bad. On my '99 XJ the PO had removed the original bad connector and jerry-rigged some wires to replace it. That all fell apart. So I did a little bit better of a rig job to hold the wiring together until I have a chance to replace the mess with the proper connector.

 

Also check the actual oil pressure sender tube, as sometimes that can clog up and throw your readings off.

Some guys will rig up a standard threaded hose nipple and a hose to the oil sender port, and crank the motor. It will spew some oil out of it, but if it is clogged it should push out whatever is clogging it.

Obviously check and replace oil after doing that, but its worth a shot. Some guys will take a pipe-cleaner and try to poke around the sender port before doing that.

 

This is what caught my attention. If you have the same results with two mechanical gauges, you may very well have low oil pressure.

The oil pressure spec for the 4.0 is not less than 13 psi at HOT idle (approx 750 rpms) and between 37-75 psi over 1600 rpms.

It is possible (and the 2 mechanical gauges show this) that you truly have low oil pressure.

Keep an eye on your coolant reservoir. Are you using coolant? The #1 symptom of a cracked 0331 cylinder head (which yours came with from the factory) is unexplained coolant loss. And if it isn't caught soon enough, it can lead to low oil pressure. I'm not saying that is the case with yours. But I'm telling you to keep your eyes open to it as a possibility.

 

I don't think it's the wiring or connector as the other bad gauge reads a solid 10psi all the time, and the one in there now is a flat zero. I did poke out the port last time, when I got a good reading with the mechanical gauge, so I don't think that's it, either.

If not just a bad batch (production run) of sensors, I wonder if there's anything that could be wrong with the engine that could cause the sensor to fail?

I'm going to save the one that always reads 10psi, though. I figure if I can't fix it and I have to go to a permanent mechanical gauge, the 10psi one will at least keep the "check gauges" light from illuminating.

 

Neither of the mechanical gauges read zero, like the current electric one does. Before it failed, it was reliably reading a varying pressure, if not the same as the mechanical one at least something. When it reads an unchanging dead zero (or an unchanging 10psi even if the engine isn't running) that looks like a bad sensor.

On the other hand... there is some milkiness to the oil accumulating on the bottom of the oil fill cap, indicating moisture, but not on the dipstick. I'll look at the coolant level tomorrow, too.

 

Modern oils have to have a whole lot of coolant/water in them to turn milky.

OTOH, the stuff under your fill cap can happen during winter short-trip operation (less than 30 miles).

 

Well, that's reassuring... 5 miles each way to work, no long drives since I bought it.