Where is the themostate at on a1988 Chevy truck it in radiator cap?

The thermostat on a 1988 Chevy truck is not located in the radiator cap. The thermostat is located inside the engine, usually in the engine block or the intake manifold, where the coolant flows from the engine to the radiator. You'll need to consult a repair manual specific to your 1988 Chevy truck engine to find its precise location. The radiator cap simply releases pressure from the cooling system.