My lady's car is coming over and needs water (and antifreeze) Someone told her to add water while the car is running but I thought the radiator cap was under a TON of pressure and this would be a dangerous thing to do. Help plz, i already forgot what I learned in auto shop 4yrs ago.

 

This is, of course, a little complicated but the answer is:

DON'T OPEN THE CAP WHEN IT IS HOT or less than about 4 hours after you turn it off.

That said, the advice was good. Starting with a cold engine, you can remove the cap, start the engine and fill the radiator. This helps to remove air and also helps to ensure a good fill. It also takes about 15 minutes of patience because the thermostat must open to finish the job. It can also be messy. Keep in mind too, that some cars use bleed screws to let air out of the system.

But the cooling system, like the brakes, is a sealed system. If you must add coolant to the radiator itself, rather than just a little to the overflow tank, then it is probably leaking somewhere. Likely places are water pump (look at the bleed hole under the shaft) hoses (including heater hoses) and, worst case, cylinder head, which shows as white smoke in the exhaust. If the overflow tank is overfull, replace the radiator cap. And don't forget to use a 50/50 mix of water and antifreeze. If she's using only water, this can also be the reason for the leakage. It's boiling/steaming out through the overflow to atmosphere.

 

What he said.

 

Turn your heater on full blast - this will *open* the system