is thermostat bad, coolant temp sensor, or something else?

Question

99 Explorer AWD v6 4 liter overhead cam
When i first start driving on cold engine when i start getting over 50 mph temp gauge goes all the way up sometimes into danger zone then comes back down to middle. After this when going 60mph or above creeps slightly higher. if crank heat comes back down (haven't left heat off it is winter). When driving 40mph or less always seems to stay normal (heat usually on lower), same when left in park and idling. I drive this vehicle for my job at typically speeds of 35mph or less with heat on low and haven't noticed problem despite driving upwards of 8 hours. I'm hoping and leaning to thermostat. Need some more opinions/advice. Please help.

mobile auto repair (mr fix it) · Accepted Answer

usually this indicates low coolant ,, i am sure you have ruled that out ... the thermostat is the next most likely cause , you may wish to replace the sensor anyway  , just so you know its working correctly

? · Answer

you are able to rather attempt the the sensor by removing the it and connecting a volt/ohm meter to the two leads and set the meter on ohm then place the sensor in a blowing pan of water.whilst the swap interior the sensor close you ought to get a analyzing of .000 too .003 if no longer it is undesirable.

Pilsner Man · Answer

If it climbs up once very high, then operates pretty much normally after that, it is a sign of a sticking thermostat. This isn't good for your aluminum heads, get it fixed.

Mike · Answer

if it doesnt do it at low speed then id have the cooling system flushed and refilled and maybe have the water pump checked. a water pump doesnt have to leak to be bad

is thermostat bad, coolant temp sensor, or something else?

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