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I’ve noticed a few times after running the truck all day that the lower radiator hose is cool to the touch, with engine at full operating temp. Also during the day my heat will go from hot to luke warm. Temp gauge hasn’t come up above about 175-180 unless pulling a big long hill then maybe it’ll hit 190. Symptoms of a bad thermostat sticking open?

CM870 coolant temp should be regulated at 185-F. If it is running very much lower than this, then the thermostat may be acting up or wearing out.