Have a '91 4.9 inline 6 cylinder. Temp guage is "normally" between the "o" and "r". A couple of days ago the guage rose to the "m". I tapped on the dash board and the guage returned to the "o" and"r". The next day when the outside temperature was about 100 degrees, the guage registered "o" to "r". This morning it rose to about the "m" again but returned to the "r" range. Do I possibly have a bad temperature sending unit? Is it located on top of the engine beside where the top end heater hose goes in? Or is there another sensor that goes to the temperature guage?

 

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