Engine temp showing low

[Stephen M]

I have a Yanmar 4JH2-TE and the running engine temp is showing 120 deg F. My heat gun is showing 160 deg F.
I would like to get advise on where the problem may be; Temp sensor or the Gauge?

 

[jibes138]

Try removing the wires from the sensor and the gauge and cleaning the contacts, apply some dielectric grease and reassemble. It could just be a bad connection. The sensor creates a voltage if it's a thermocouple or a change in resistance if it's a thermistor and excess resistance in the circuit will result in a bad reading at the gauge. The sensor doesn't have moving parts so failure is less likely than the gauge IMHO. The sensor uses the threads of the housing to complete the circuit and if the threads are corroded or someone used Teflon tape on them you could have a bad circuit there. Here's a troubleshooting guide. How to Test a Water Temperature Gauge on a Boat

 

[MikeHoncho]

Check the associated ground. Try increasing the electrical load on the system and see if the temp changes with the load change. I've denn this with bad grounds.

 

[Stephen M]

Thanks for the great advice.