instrument gauge cluster issue?

I am not sure if I have a gauge, wiring, or sending unit problem. I am looking for any additional troubleshooting steps I can perform to narrow down the problem.

On my 97 XJ,

1. The oil pressure gauge will read as low as 10-11 psi at hot idle. This seemed too low, and I put a mechanical gauge on it and drove around for a while. The mechanical gauge was reading about 17 psi at hot idle. The sending unit I've got in there is a new NAPA Echlin. The original sending unit wouldn't read past 40-42 psi and would also read on the low side, so I bought the NAPA Echlin. The NAPA Echlin one seems to go up to 50 psi, but both seem to read on the low side at hot idle.

2. The coolant temperature gauge may read a little high at hot idle - halfway between the 210 mark and the next higher one. This seems like it could be the fan clutch, or even some airflow issues with the condenser. However, with an IR thermometer pointed at the thermostat housing, the thermometer doesn't seem to show the higher temperature.

Do these issues sound like a sensor, wiring, or gauge cluster problem? I am at a loss on how to test any more without swapping parts.

It is recommended to purchase the sending unit directly from Jeep. It's about $20 more than aftermarket, but will perform better and last longer. I too purchased one from NAPA only to have it fall apart in my hand during installation. I was also getting readings around 10 psi at hot idle on my 98. I purchased one from Jeep and it has been performing well for 2 years now (20 psi hot idle and 45-50 psi cold or under load). Hot idle spec on the 4.0 is 13psi at idle and it sounds like your mechanical gauge readings confirm it is a sending unit issue, and not an oil pump or other internal engine problem, although 17 is not excellent.

Your coolant temperature gauge is also not known to be very accurate. You can pull the connector and gently file the contacts to make sure you're making a good connection. This solved my gauge cutout problem several years ago and has not returned. The E-fan should come on around 215 degrees, although I'm not sure on that exact temperature. Is your E-fan coming on at all? Again, your testing with an IR gauge points to a less than accurate gauge reading. A bad fan clutch will cause a temp creep at red lights. Since your IR gun doesn't show a temp increase beyond the gauge I don't think you have a problem.

If you wish to proceed further, what oil viscosity are you running? What type of oil filter are you running? When was your last cooling system flush? What mix of coolant are you running? Basic tune-up info can help fine tune your 4.0 if it's bothering you.

Yeah, I agree it could be the sending unit. Its the NAPA Echlin, not their Mileage Plus Electrical, so I expected it to fix the gauge reading. I have a NAPA Echlin on my 01 and its fine, but I can't swap to test because 01 used a different sending unit. I have 10w-30 conventional in it right now with a NAPA Gold 51515. It has 184,000 miles indicated but may have around 190,000 or a little more because of larger tires. The engine runs fine and has plenty of power.

The electric fan does cycle at the 'right temperatures' as far the dash gauge is concerned. I did spray contact cleaner on all the connections. I could swap sender with my 01. Since this 97 is new to me and the radiator looked suspect and the radiator hoses weren't exactly right and rubbing, I went ahead and changed everything... Auto Zone Spectra radiator, dealer thermostat, dealer water pump, dealer hoses (all), NAPA thermostat housing & radiator cap. I run pretty much the same on my 01 except the thermostat housing on that one is a new dealer and has a new NAPA fan clutch. Antifreeze should be 50/50 Zerex conventional green and distilled water. I poured 3 gallons of antifreeze through the heater core supply and let it come out the water pump inlet. Then I changed all the parts. Added 6 quarts pure antifreeze and topped off with distilled. Put 50/50 in the expansion tank. The temperature reading is the same before and after the work so I don't think I did anything wrong.

I guess I was wondering if there is a chart for ohm or voltage readings I could compare against to isolate the gauge or the sending units.

The cheapskate's solution to this is to make sure you know what a normal hot idle pressure is. It doesn't really matter what the gauge says as long as it's working and changes when pressure changes. What you really need to do is to make sure that you notice anything abnormal. The gauges are pretty sloppy.

I have not had a post-97 set apart, but I had a problem with the voltage gauge on my 95, and found that the gauge is calibrated, essentially, by a glued-in magnet, which can come loose.

It's been a while since I checked my FSM but I have one for the 99 that I think has charts for the gauges. I'll try to find it and check back later.

I replaced the NAPA Echlin oil pressure sending unit with one from the dealer and it seems like the problem is gone.

I replaced the fan clutch with a NAPA unit and it seems like it solved the warm running at low speed.

I don't think replacing the fan clutch had anything to do with the oil pressure readings...

I'll have to see if NAPA will give me my money back on the sending unit.

Tell them that it did not function correctly and you bought one from the dealer.
However, yes, the dealer sensor may "work better" But factory gauges are know to be innaccurate.
As far as your coolant temp. especially since you stated your IR reading was lower than what the gauge said. Know that when you drive it.

NAPA said they'd refund the part if I had the box.

Otherwise they'll just swap it.

Got to love corporate policies.