Possible faulty coolant temperature sensor or a short circuit in the wiring harness would be my first guess.
Try disconnecting the coolant temperature sensor plug and see what that does to the gauge.
Oh and treat the garage advice with scorn,
What the hell the dip stick or oil cap has to do with engine temperature beats me,
unless the engine has actually melted,
Posted on Saturday 9th of January 2016Don't understand this? Ask a question