1998 Frontier possibly overheating

2.4, automatic, 134xxx miles

So I think I have combination of problems. My old lady neighbor across the street brought her truck over yesterday because she said it washttps://imgur.com/a/RorcSSW overheating. I checked everything out on it, she had replaced her radiator cap because it had been leaking but the overflow tank was full, radiator was topped off, cooling fans operate, no leaks, and it never got hot while idling or driving. I noticed the coolant was a deep brown so I drained and flushed out what I could, refilled the system, bled it, and returned it. She came back today saying "it's hot right now" and it was. The temp gauge wasn't fully pegged but it was up there, I shut the key off and grabbed my scan tool, it read 190. In my understanding, 190 is perfectly fine but this temp gauge was reading otherwise. I also checked the temp of the upper and lower radiator hoses and found the top to be 185 while the bottom was 105. I've found out that there's a separate temp sensor for the gauge and the OBD, is it likely the sensor for the gauge? I'm fairly poor with electrical skills and aiming to improve. The gauge doesn't behave erratically like it's shorting out, it moves slowly and moves up and down with what my scan tool reads, but it's like 190 is nearly max temp according to the dash gauge. 185 will drop it slightly higher than the halfway point. It's like the value of the needle is just higher than it should be. 

Separately, the large temperature difference between the hoses, is this likely poor flow? Does this indicate I may actually be overheating and maybe the OBD sensor is incorrect?

I've found out that there's a separate temp sensor for the gauge and the OBD, is it likely the sensor for the gauge?

I think you're right to suspect the gauge sender, given that the ECT sensor is reading rationally. Before replacing the sender, you can prove out the instrument cluster and the wiring. Key on, unplug the sender and short the wire to ground. The temperature gauge should show full hot. Then, take the short away and leave the sender circuit open. Should drop the temp gauge to full cold.

If you can force the gauge to swing full range, then I'd say it's sender time. If you can't, then there may be something wrong with the cluster or wiring.

Separately, the large temperature difference between the hoses, is this likely poor flow? Does this indicate I may actually be overheating and maybe the OBD sensor is incorrect?

Are you confident that your reading of 185* was valid? Because the parts catalog says the OE thermostat opening temperature is 170*. Those temperature readings might suggest a stuck (or intermittently sticking) closed thermostat. Which radiator hose is connected to the thermostat?

The easiest way to check the ECT is to let the vehicle sit overnight and compare the ECT to the IAT first thing in the morning, before startup. They should be within 5* of each other. If they're close, the ECT is likely working correctly. If the difference is too big, one or the other may be lying. Compare the readings to actual ambient temperature and see who's closer.

Thank you! I took her truck back across the street to her tonight because she'll drive it a half mile down the road in the mornings for breakfast. I made sure she knows not to go anywhere else

But yes I am positive of the 185 temp reading. Actually did triple check both sides because I was shocked by the difference. Upper radiator hose runs to the thermostat.

I'll get her car from her after work tomorrow and let it cool off into the night, then check the ECT and IAT for that comparison

Tonight's temp according to my phone is 50 degrees. IAT read 55 and ECT read 57. Shorting the wire for the gauge sensor does in fact send it to full hot, 3 times in a row. And it drops to dead cold with the ground removed. I think I'll replace this sensor and thermostat and report back any changes

Got around to installing a new thermostat and temp gauge. Temp gauge will now fluctuate cold to mid on its own while engine is cold and key is on but very intermittently. This may have been a problem before but I just never saw it.

Engine running, the OBD ECT raises very normally to a comfortable 192-196 and stays there even with heat on full blast. Radiator hoses read ~153 on the top and ~89 on the bottom. Heat in the cab is perfectly comfortable and reliable. Overfow tank sucked some coolant down but I believe all the air bubbles are out.

The dashboard temp gauge fully pegs out by the time the OBD temp reads about 180. It starts reading hot around 160. I'm suspecting the dash gauge at this point but also wondering if her radiator is plugged up since the hoses have such different readings.

Did you ever fix this car?