Is it possible the thermostat was installed backwards, with the temp sensing mechanism on the radiator side? Could explain the engine getting hot - very hot - then once the thermostat does start to open the rush of very hot fluid past the mechanism would force it open very quickly. As you are driving, of course, the thermostat would operate normally.

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That sounds very logical. I would check on that first. Also check if the fan stops almost inmediately after shuting the engine off. If it does, the fan clutch it's good.

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Juan
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