even if you have a perfectly good working thermostat, but if the system has an air lock in the engine block and cylinder head, the thermostat will not open because there is not enough antifreeze to trigger the thermostat to open and circulate the antifreeze all around the entire cooling system, therefore the bottom of the radiator will be too cold compared to the steaming hot upper radiator temp. be sure to burp the system and get rid of the air inside the engine block and cylinder head to ensure the proper operation of the thermostat and the circulation of the antifreeze over the entire cooling system.

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'85 300D Turbo - CA Version