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Open your bonnet after running your engine and turning it off and CAREFULLY (IT CAN BURN YOU) unscrew the cap<<<
DO NOT TRY THE ABOVE if the engine is up to norm temp....
When the engine is up to running temp the coolant temp is ABOVE boiling point. Undoing the cap lowers the pressure in the system allowing the fluid inside to boil, and it then comes out of the filler cap like a volcano!! It will hit the raised bonnet and bounce forward all over you....... I saw this happen, the guy was in hosp for a couple of days, and still has scars on his face.............
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