In order to cool down the overheated engine, it is recommended to put water slowly and carefully. There are two reasons for the same. One is to avoid engine to crack down and second reason is safety. If overheated engine parts are cooled too rapidly, then there will be uneven thermal contraction in the parts. As a result, thermal stress is developed and potential cracking of the head and/or block may happen. The blow back pressure of putting cold water in an overheated engine can cause severe burns.
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The density of waHC Verma - Concepts of Physics Part 2