I had an issue yesterday with the temp gauge instantly rising to over 250 degrees. We deemed it to be the fluid temp sending unit to the gauge. I drove the car for about 30 minutes to a shop to get the work done. When we popped the hood just after I got there and took off the pressure cap on the passenger side coolant reservoir. It wasn't hot at all. Warm at best. Both of us thought that wasn't right at all. How hot should that coolant be in the resevoir? We're afraid that we have a thermostat that is stuck. We removed the temp sending unit off of the drivers side in front of the cylinder head and checked for coolant flow through the block and it was good.