Adding coolant to a hot engine

Many years ago my dad told me never to add cold water to a non-running hot engine.  My first car was a clunker and he said if it ever ran low on coolant and began percolating, only add to the radiator while the engine's running or I would risk cracking the block.  He also warned me about removing the cap when it's hot, too.

I've always taken that advice and, thankfully, never had occasion to that circumstance - but was he right?
Ever heard of someone having a block crack, or popping a core plug because of hose water into a hot engine?

Never have.

Ive taken full operating temp engines before pull the upper radiator hose off stuff a water hose in the radiator and crank it up letting the water hose keep the radiator full to flush the system out with cold water.

I think the only way youd have a problem is if you had your engine severely over heated and attempted to throw ice cold water to it.  Adding water to the radiator wouldnt cause this cause by time the water got to the engine it would have been warmed up by the warm/hot radiator to the warm hoses and water pump.

I'm sure you're right, Rusty.

Makes more sense the added water or coolant would be plenty warm before it got past the thermostat.

Yes I have also hear / told the same thing so I dont add cold, cool maybe but not cold water to a hot motor.
Now the thing is I NEVER OPEN A HOT COOLING SYSTEM for fear of getting burned from hot coolant blowing out so the motor would be pretty cool before adding anything to the radiator anyway.

I wonder if it had something to do with how the blocks & heads were cast back in the day?
Then again I have had the heads on my 02 Dodge show cracks when pulled and it never over heated and I have had the truck since new.
Dave ----

I can't imagine ever getting enough cold water into a hot engine quick enough to damage it. Engines are much tougher than people give them credit for. Think of all the people in Canada (northern, or western) and the Northern USA like Montana or Minnesota where they see -40C/F in the winter and start their engines...starting compressed explosions inside an engine block that is at -40.

Rembrant wrote

I can't imagine ever getting enough cold water into a hot engine quick enough to damage it. Engines are much tougher than people give them credit for. Think of all the people in Canada (northern, or western) and the Northern USA like Montana or Minnesota where they see -40C/F in the winter and start their engines...starting compressed explosions inside an engine block that is at -40.

Not to mention the ice road truckers that do the hauling on frozen rivers and lake beds.  Their engines are hot but their radiators are still with blankets blocking the air flow going to be pulling in air that is obviously below freezing.