Have never had a digital temp gauge on a vehicle until now. Last night fired up the truck and within seconds the temp gauge read 236 degrees and popped a light, then immediately dropped down to 220 and then fluctuated between 203 and 235 all the way home about 30 miles going slow. And by fluctuating it was jumping the whole range both ways in seconds. Analog gauge never wavered off of middle and engine never gave the hot shakes, never smelled coolant, or acted like it was hot in anyway. But got home and radiator hoses were hotter than normal so there was definitely an issue. Could it be just the sensor or thermostat about to go? Or both?

Would help to know the year. However, I'm going with faulty sensor. Inexpensive part. Slap it in yourself.

Yeah year might help.2014 with 126k on it mileage wise.dont get out much anymore in it. Was bought TN 2019 for pulling my camper ith 92k on it.

Years ago I had a thermostat getting stuck open in a 2002 vehicle. I was driving on the highway and coolant temp was running way cold. Replaced the thermostat for $12. It was a cheap part and easy to swap.

It could be either one. I'd replace the cheaper one first. Not sure how to easily diagnose without just changing a part unless you can dip the sensor somehow in ice water and boiling water to see how accurate it is.

This is all in a 2014 Dodge 2500 HD 4x4 with the 5.7 litre Hemi.

Ok so replaced the Thermostat, Housing, and sensor and the problem I was having went away, 2 months later almost to the day it happened again, was not as consistently running hot, but once again the coolant temp spiked to 250 and needle gauge spiked into the red and idiot light came on. Immediately shut it down and no other symptoms, no steam, no smell of burnt coolant or coolant at all, listened at radiator and heard no bubbling or boiling. When restarted temp was down, the needle gauge was at 3/4, if think of it like a gas gauge, full being hot, and digital was saying 224. When I shut it down digital said 248 and needle had spiked to red. Limped home and never spiked again, and actually under load the temp dropped and when not under load temp started back up but with heater running and going easy never went back over 235.

Went out to it to do some testing today and it fired right up and no issues. Got a half mile down the road and stopped at the intersection and temp started back up while waiting for traffic. Pulled onto main lane to do my testing loop and temp dropped back down to normal levels once moving very quickly, but as settled into no hill no real load on the engine it started back up. Never let it get above 235, but would creep up and then creep back down. Turning on heater made for a drastic drop in temp, then would creep back up when turned off, and then when turned back on and left on it started to creep back up but much slower while heater was on. Oil and Transmission temps stayed within their usual levels, as did oil pressure yesterday and today as well as the last time I had this issue.

Could this be the water pump trying to go out? Could it be air trapped in the line? Something more insidious like a head gasket leak? Oil looks clean and there does not seem to be any loss of power indicating a possible mix of coolant and interior engine pieces and parts and fluids. I did notice a distinctive whine but not a whine when the fan kicked to "High Speed" as the temp hit 220 then it would kick down and the noise was gone once the temp dropped back down. So maybe an issue with the fan/fan clutch?

Fan running when temp first spikes?

TexAg1987 said:

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Fan running when temp first spikes?

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It is but at a slower speed and no "whine", then kicks to higher speed at 220 and the whine starts, stays on at that speed through any spikes and then slows back down once below 220.