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I suspect that someone has removed the thermostat from the water inlet housing on the front of the intake manifold. To check, take the two bolts out of the housing and lift to see if a thermostat is there. You can drain some antifreeze/water out of the radiator before, if you want, or just loose a bit when you open up the inlet housing. You will need a 160 deg. or 180 deg. termostat (there is lots of debate as to which is best...180 came standard on the car) if you don't have one...you will also need a new inlet housing gasket and some Permatex sealant for the bolt threads and the gasket when you go back together.
Okay, finally had the time to change the thermostat
There was one in there but i replaced it with a 180.
The gauge goes up as pictured-still seems low to me, but maybe I am wrong
The pic was taken after a full warmup.
What do you think?
I have a 70 vert 350/300 and I could take the very same picture. I once had a hot car but since adding rad seals (foam seals) and new rad cap the car is very cool. I think if you get the air to flow through the rad as designed the vette can be a cool running car. Makes me wonder if this is too cool.
My 71' doesn't get much hotter either. No A/C, new rad and seals must make for efficient cooling. Check the actual temp of the water in the rad to see if it's anywhere new what the gage says. Be careful, it's HOT.
FWIW, I have the same exact situation. I've replaced the sending unit and the thermostat, 180 degrees. No change. Yesterday was a very hot day here in VA. I put about 60 miles on it and the temp never indicated above 155. I got home, popped the hood and noticed I was leaking fluid out the overflow tube. Probably lost about 3-4 oz. Nothing terrible, but that doesn't seem normal.
Appreciate anyone who can make a recommendation as to why the gauge only goes to 155.
I have posted this problem many times, when I ground out my sender lead the gauge only goes to half way max. I am currently just living with it so that if my gauge gets past the first mark....it's getting hot.