What could this be? Thermostat?...1969 350 300Hp Chevelle Sport Coupe...Please advise .......Checked fluid in Radiator and was Full and Green......Where do I start?:beers:

Put a temp guage on it and see if you are overheating. Your temp sending unit might be bad.

Please explain alttle more...Kinda little rusty on my automotive skills...It's been awhile...Thank You

As was mentioned it could just be a faulty sending unit.The only way to tell for sure would be to install an aftermarket(cheap) temperature gauge to see exactly what your temps really are.If you didnt want to leave the gauge in permanently you could just mount it loosely inside the car to monitor temps while you drive until you determine what your temperatures really are doing. If this sounds like too much trouble then you could simply replace the original sending unit to see if that cures the problem:D .

Another way to use an external temp gauge. The kind you point/press button and it will tell you the temperature. They are fairly accurate. Mine can accurately read the temperature of the heads between the exhaust manifolds when the engine is running and hot. Move it an inch to the right or left and it will tell you the temp of the exhaust manifolds. Perhaps a friend has one (HVAC guys sometimes have them or building maintenance people).

Try to define WHEN it goes on, also. For example, only when sitting in traffic, only on the highway, etc.

Engine could actually be overheating, which can be caused by a variety of things - more info would help.

FYI, sensor will have a single wire coming from it, and be either on one of the heads or on the intake manifold.