Leakage around the t-stat?
For the cooling system to reach normal operating temperature, the entire thermostat assembly must completely shut off the flow of coolant to the radiator.
Even if your t-stat
valve is checked in a pan of water and found to be working correctly (stays tightly closed until rated temp is reached), it doesn't guarantee that the entire
assembly is tight.
It is common to find leakage past the thermostat flange. The flange is supposed to be clamped between the recess in the intake manifold and the water outlet - which would create a water-tight seal.
But sometimes, due to a number of factors, this seal is lost and engine coolant is allowed to flow into the radiator as soon as the engine is started. This image shows a different type of t-stat but the idea of flange leakage is the same:
You must be registered for see images attach
If the leakage is severe enough, the engine coolant cannot reach design temperature.
Here are a couple of things to check:
That the landing area in the intake manifold is clean and not excessively pitted:
You must be registered for see images attach
Also that the water outlet housing is securely clamping the t-stat down in the recess of the manifold. Pull the upper radiator hose and check for looseness with a screwdriver:
You must be registered for see images attach
You could also pressure test the thermostat:
Pull the upper radiator hose from the t-stat housing - leave the other end connected. Make up a rig that will allow you to apply compressed air to the open end of the hose.
Press up the cooling system with just a few pounds of air (2-3 is plenty). Spray soap suds down into the housing and watch for bubbles.
