Engine only warms up to 140

[Snakebit]

OK. If I let the car sit and idle the temp goes to 180-200. If I drive the car at 50mph and the temp does not go any higher than 140. I blocked off about 1/3 of the radiator with a piece of cardboard and it stays at 160. March street pulleys: crank pulley is 5 1/2" water pump is 5". Under drive water pump pulley would be 6". The robert Shaw thermostat has (3) 1/16 holes drilled in it. The thermostat should be closed at 140, so how is the engine staying at 140? There should be some fluxuation if the thermostat is opening and closing.

[vettestr]

I am not convinced the system is not being bypassed and defeating the t-stat. The tests in post #19 will confirm or deny that fact.

[undy]

..gut feeling is the t-stat is not seating closed well or those 3 holes drilled in the t-stat are a wee bit larger than 1/16".

Under cooler ambient conditions you may be getting too much coolant flow through those 3 drilled holes or an unseated thermostat, more than the engine's heat output at that time. This holds true especially under low power cruise conditions where there's minimal throttle action. The engine's heat generation is somewhat proportionate to horsepower output. My money's on a defunct thermostat. Let us know what the end result is..

[jhv48]

Is your fan turning on at the proper temp? Or is it running all the time?