Coolant temp gauge starting to hunt around during cold weather

I don’t think a thermostat would cause your issue. They usually fail stuck open or closed. Maybe the viscous fan drive is nearing its demise?

Do a

Alex05 said:

Maybe I'm just eagle-eyeing it now, but I have been noticing my coolant temp gauge hunt around on either side of the '210' dot. Once at operating temp, the gauge can't seem to decide where it wants to land. It will move from just below the 210 dot to just above it during normal driving. Actual temp seems fine; There are no obvious physical signs of overheating.
Of course, this gauge is mostly a "OK/Not-OK" gauge. Before I put a scan tool in my passenger seat and watch temps while driving, has anyone else encountered this issue? Was it actually anything to do with your thermostat?

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After you are finished investigating and ruled out/confirmed the problem is with the thermostat or radiator cap; you may need to move onto electrical problems.
Do any of the other gauges fluctuate in a similar manner when the temperature gauge is fluctuating ?
Are any other electrical items affected in the cab ?
Check the grounding stud connections (oxidation and tightness) near the oil filter and the grounds that are attached to the screws for the front speaker mounts and beneath the dash panel at the side kick panels.

Ive had a few vehicles that constantly cycled temp wise in cold weather only. The 3, 1st gen Ram diesels we burned thru were worst. Normal or good, probably not. it cycled with big enough changes at times I figure any day now Id be replacing a cracked head or block. Stayed that way for years in cold cold weather only. I attribute it to crap thermostats as I wasnt gonna spend $100 for the Cummins one. Lesson learned.

that's completely normal. You're just seeing the operation of your thermostat.

Your engine gets warm (needle ticks just above the 210 mark), thermostat opens, a bunch of cold water that had been sitting in your radiator works its way through the engine and hits the thermostat (and sensor) before reaching 210...thermostat closes, coolant warms, thermostat opens, and the cycle continues. It's more pronounced in colder temps because what's sitting in the radiator is much colder now than it would be if it was 85 outside.

Once the speaker grills are removed; the ground wires are under the screws fastening the speakers to the dash.
There are also grounds beneath the dash attached to the side kick panels.

If the cooling system isn't exactly up to par the thermostat will be mostly open. The thermostat operates very smooth when it's in the normal range. Put one in boiling water and watch. it also is sitting in coolant so it responds quickly to temperature change. What doesn't is the fan clutch. The radiator has to start dumping heat for it to operate. After the radiator gets very hot that air starts to spin the fan faster and the radiator cools down. The fan clutch has quite a bit of lag compared to the thermostat. Long story short if you experience slow temp swings it often points to a general reduction in cooling capacity due to rust and clogs and other crap leaving the thermostat mostly open and the fan controlling the temperature. Just food for thought. Some people like to dispute just about everything because it's the internet, if you disagree I don't care, please don't quote me.

Alex05 said:

What is a viscous fan drive?

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That's your fan clutch. It varies the fan speed via its silicone liquid to cause the fan to spin faster in hot conditions and slower in cool conditions.

Your temperature fluctuations seem completely normal to me.

Ok but it sure sounds normal to me, mine does it too.

Hello, another inquiry from Hungary. The coolant in my Jeep usually doesn't reach operating temperature. New Thermostat Gates 195 round 91 Celsius. Okay, it's winter now and the outside temperature is zero degrees. Despite this, I can't get the operating temperature to exceed 40 degrees Celsius whem I ám driving. Is that normal? What about the radiator fan? I have the feeling that it's running all the time. I know this from other cars, where there's a slip clutch and the radiator fan only runs when it gets too hot. Is that the same with the Jeep, or does the radiator fan have to run all the time? Could this be due to faulty electronics or a broken relay? I've now had five thermostats and four water pumps; all work was carried out in a workshop, and that's perfectly fine. I remember that the operating temperature was around 90°C last winter. For about a year now, the engine has not been getting properly warm and only rarely reaches 90° Celsius. What could that be?

The 4.0 uses a thermostatic fan clutch. No electronics.

Normally the temp gauge will run around 210 F, or 100 C.

The can seize, which means it doesn't slip anymore, it becomes a direct drive fan. It seized, it will pull too much air through the radiator and put quite a bit of drag on the engine, resulting in poor fuel economy.