Question About Oil Pressure Sending Unit and When to Replace

I have a 98 XJ Limited 2wd with the AW4 and I've read most of the older posts on faulty oil pressure sending units. My question is to help me avoid throwing new parts at a problem when I'm unable to diagnose my way to a solution without spending money. The oil pressure gauge in my Jeep runs on the low side (like a lot of them from what I read) I have 144k on my XJ and I belive the sending unit might be failing. I have zero knocks or noises from the engine bay and the Jeep is a daily driver. I just get nervous when it shows 10-13psi at warm idle. I get about 35-40 at cold idle. I tried to take everyone's advice and rent a mechanical pressure gauge from my local parts place to verify the sending unit is heading south. When I asked the parts person if they rented these gauges he asked me what the gauge was for....and no, they don't rent them. So my question is......should I spend $20-40 on a mechanical gauge to test my condition or just bite the bullet and order a new sending unit from Jeep for $52. I'm not about to throw parts at it, but I'm fairly confident this is the issue and I don't want to spend money on a mechanical gauge only to have to spend more money on the sending unit. Any thoughts? Thank you in advance.

Those numbers look fairily normal. A mech gauge is always nice to have as well.

Does the needle on the guage jump around at all?

Often when the senders go bad the needle on the guage will jump around or peg-out, especially when accelerating from idle.

I've seen them behave like this randomly or intermittently, as well... good luck.

It does bounce a little bit, but not excessive. I notice varying readings.....for example, one day the hot idle will show 10-13psi and I'll drive for a few more miles and the continued hot idle will then read 15-20psi. The cold pressure fluctuates in the same manner....some mornings it will read 40psi at startup and other mornings 30-35psi at startup. The needle never pegs or bottoms out and I never get a "check gauges" warning. I take good care of my XJ and don't want to miss an early warning sign. In a perfect world I would fill my garage with tools for her but money is tight and I can't afford to do both. I can either buy the mechanical gauge and hopefully determine I have a bad sending unit or buy a new sending unit and gamble. Another piece of info that might help with suggestions.....I tried to take the economical route about 3 months ago and bought an aftermarket Borg Warner sending unit for about $23. It literally fell apart in my hands while trying to install so I installed the original one back. It's not leaking but I'm thinking its days are numbered.

There is no right or wrong.

Some guys check pressure with a mechanical gauge, some guys just swap out the sending unit.

What I like about the confirmation with a mechanical gauge is that it IS confirmation of your actual oil pressure. And then you have that tool for future use.

If you buy a sending unit and your pressure still reads low, you'll wish you had gone the other route. I'd say confirm it. Know that the gauge circuit can have some inaccuracy in it as well. Mechanical gauge will settle that doubt.

Spec. oil pressure for the 4.0 is at least 13 psi at hot idle, 37-75 psi over 1600 rpms. But with that said, I've seen guys with 10 psi of confirmed oil pressure at hot idle run for YEARS like that.

Got Harbor Freight?

http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=92687

PS--don't you just love it when some counter-prick has to ask "what for" when the a** knew they didn't have what you wanted anyway?

PPS--did a search on Vatozone for mechanical oil pressure gauge--93 hits, four of them less than $20, if you don't have Harbor Freight make your own.

I would get the mechanical gauge. Once you have it, you'll have it forever. Think of it this way: once you know what the normal running pressure of your engine is, it doesn't matter if the sender is inaccurate as long as it's consistent. All you need it for is to monitor deviations from normal. It doesn't matter what the number is, as long as you know what's normal and keep an eye on it. (edit to add...besides, it might be the gauge that's off, not the sender!)

By the way, don't forget the effect of a good oil filter on those readings. If you've been using filters of unknown provenance, get a Wix for your next oil change, and it might come up by 2 or 3 pounds.

98NWCherokee said:

I have a 98 XJ Limited 2wd with the AW4 and I've read most of the older posts on faulty oil pressure sending units. My question is to help me avoid throwing new parts at a problem when I'm unable to diagnose my way to a solution without spending money. The oil pressure gauge in my Jeep runs on the low side (like a lot of them from what I read) I have 144k on my XJ and I belive the sending unit might be failing. I have zero knocks or noises from the engine bay and the Jeep is a daily driver. I just get nervous when it shows 10-13psi at warm idle. I get about 35-40 at cold idle.

Click to expand...

When the oil sending unit on my 1999 Limited failed, the gauge showed crazy stupid high pressure, like 80+ and stayed there. Replaced it with an Autozone part and everything is okay.

Sheeat--if you've been running a Fram filter, you might see the #'s jump by 10psi if you switch to a WIX, mobil, K&N(mobil), or NAPA(wix).