My radiator cap was loose and my car engine ran hot. Once I tightened the radiator cap the car stopped running hot. My question is, why would a loose radiator cap cause my cars engine to run hot?
The boiling point of water is 212 F @ sea level. Your cars electric cooling fan won’t come on till 225 F. So since cars cooling system is not pressurized (loose radiator cap) the coolant would start to boil around 212 F.
The coolant never became hot enough to kick on the cooling fans, but hot enough for the coolant to boil and the temperature gauge to read hot on your dash. Once you tightened the radiator cap on the radiator (or recovery bottle) the coolant was able to reach operating temperature to turn on the cooling fans to cool the engine down.
Now I recommend that you change all your coolant to make sure that you have the right 50/50 concentration of distilled water and antifreeze. Winter is right around the corner and weak neglected antifreeze can do a lot of damage to your cars cooling system and engine. Finding shop that can do it correctly with a coolant exchange machine will save you a lot of money down the road.
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