I have a fairly new 180 degree T stat in my 73 240Z. The temp guage never goes past the middle of the guage even on the hotest south Ga days. Lately, while driving at highway speeds, the temp guage will go back to zero. A few minutes later, the guage will go back to the normal reading. I have converted the car to a closed system with a surge tank. The coolant is at the proper level when the car is cold. I have also checked the temp sender connections, and all is well. Any suggestions guys or gals?

If you've checked the connections then the problem could only be the guage or the sender. I'd kind of lean toward the sensor since I had a similar problem with the one in my 810 a few years back and it turned out to be the sensor.

IIRC ground the wire that connects to the sensor and turn the key to "ON". The temp sensor should max out. If it does not that may indicate problems with the gauge.

But as Stephen has said, it's more than likely the sensor.

2¢

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Thanks guys for the comments. I will try the ground test and see what happens. To my knowledge the sensor is original, so it is probably going bad. Thanks for the help as always.

Brad Richards