Overheating but not Overheating

[BillyZ]

I have a 2003 Merc 350 Mag MPI. The impeller is new, the system has been blown out but the temp gauge still showed it was overheating. I replaced the temperature sensor but the gauge is till behaving the same. It starts off low, slowly gets to 160-ish and then moves up rapidly to 250. At 250 I can put my hand on the thermostat housing and I have used my laser thermometer and the therm housing is around 160, the stack are 90, the block is 120.

Does a temp gauge behave like this when it fails? Is there something else I should look at?

Thanks

Bill

 

[Bt Doctur]

either a failing gauge or a bad ground on the gauge

 

[kenny nunez]

Sometimes a sending unit will act like that.

 

[alldodge]

Put the temp sender in a pan of water on the stove with an ohm meter connected to it. Measure the water temp as it rises and see if the resistance goes down accordingly

Water Temperature - Ohms Reading

75°F : 600-800
100°F : 450
140°F : 121-147
149°F : 99
194°F : 47-55
212°F : 36-41
250°F : 29.6

 

[BillyZ]

Sending unit was replaced earlier today and did not cure the problem. West Marine happened to have my exact temp gauge so I picked one up. This "seems" to of solved the problem. I can see the therm housing is running around 165 with my laser thermometer and the gauge is holding around 170. Headed out to the lake soon to give it a shot.

Thanks Everyone

Bill