I have a '91 with 23k miles. Dash coolant gauge was showing hot. Took to reliable mechanic and he determined and computer showed that the fans were coming on at the correct time and that the coolant temp per the ecm was where it should be but either gauge or sensor was bad. We replaced sensor and and the dash(oem gauge) still showed hot and it was his belief that with the coolant cap off and seeing a few bubbles flow by every 15 seconds or so and with a heat gun showing the only spot to be hot on the side of the motor where the sensor goes and that actual coolant temp was fine that it may be a head gasket, or head crack. Today I put a manual gauge on and its the same. The gauge got up to 245, a heat gun where the gauge goes in the motor shows 245 but every hose, coolant tank and other part of the cooling system that I check is all still at 180. I turned the car off before it got any hotter. The car Is pushing out zero coolant and shows zero signs of actually overheating but is hotter at the sensor location on the passenger side of the motor?

 

Are you seeing flow under the cap. With Thermostat open you should be seeing movement. Given the low mileage this car sits & sits a lot so it might be sticking shut.

 

Older Corvettes with Lower miles will do things like that. I would replace parts like the thermostat right away and flush the coolant. On the subject of coolant sometimes people get a little overboard and add too much anti-freeze which will cause the engine to run hotter. In Virginia I use 70% Distilled Water and 30% antifreeze as I need all the cooling help I can get here. Freezing is not a big issues where I am. Check your lower radiator hose to see if it is collapsing while pumping the coolant. It is important to replace old soft hoses as well or you can have a devastating overheat and warp the heads or blow the head gaskets.

There is a part called a Coolant Temperature Sensor and it's job is to measure the engine's Coolant temperature so it can make the necessary adjustments to the air fuel ratio. The CTS is on the front of the plenum near the throttle body. You need to verify that it is accurate. It has three wires and sends a resistance value back to the ECM. The value can be converted to a temperature using a chart in the factory Service manual. This sensor has to be working properly or you will have troubles starting.

On a Corvette like your even though it has low mileage all of the components on your Corvette are 30 years old. Many electrical components used in the dashboard had a 15 year life expectancy. No matter whet the mileage is you will have the same old Corvette syndrome. There are many parts on that Corvette that might have to be replaced just due to sitting around. The oxygen sensor is a great example, they were made to last 24 months or 50k miles. They CAN last a bit longer but not at their prime. To get the maximum Performance and Economy you will want a new Oxygen sensor unless you already did change it. A bad O2 will make the car run richer and the mileage will be down.

 

You should buy a thermostat outlet that has a port for temp sender and drive it a little bit.
monitor the temp. As it goes into the radiator.

also there is the water bleed at the back of the intake on the right rear of the intake manifold.
it is supposed to eliminate trapped water at the back of the engine.
you might want to check to see that that tube is not rusted shut.
also #8 is the hottest cylinder on the small block chevy, so if you move the sensor and it gets cooler, great.
you do realize there are two temp sensors.
one for U to see, and one for the computer to use as a reference.

if your temp goes high, there is a red light that comes on on the digital dash next to the temp numbers.