What to do when a car overheats?

Dear Car Talk

Dear Car Talk | Aug 01, 1999

Dear Tom and Ray:

Perhaps you can settle a discussion that my son and I had recently. When his car overheated, I suggested that he add water to his radiator since the level was low and he
needed to keep driving. He insisted that he had to drive the car to the auto parts store to get antifreeze because the radiator cap said "radiator coolant only." I told him
that water IS a radiator coolant, and that the cap was so-labeled so that some idiot wouldn't put motor oil or transmission fluid in the radiator. If you prove me right, I'll
send him a copy of the column. If not, he'll never know. -- Bob

RAY: Hope you've got a 33-cent stamp handy, Bob. You're absolutely right. Water is definitely a radiator coolant.

TOM: In fact, it was the first radiator coolant. If you watch "Ben Hur" carefully, before the big race, you can see him pouring water into the radiator of his chariot.

RAY: Ideally, antifreeze (usually ethylene glycol) is added to the water in a 50-50 mixture. The combination has a higher boiling point and a lower freezing point than
either of the two liquids individually.

TOM: But in an emergency, it's certainly better to have some coolant than no coolant. And in most cases, the coolant level is low, not empty. So there's already some
antifreeze in the radiator and you're merely changing the ratio.

RAY: So water is an absolutely acceptable way to top off your radiator if no antifreeze is available. And in fact, we've taken questions from people who have used all
kinds of water-like substances in their radiators in emergencies. And in most cases, it was the right thing to do (we're still not sure about the Mountain Dew.

TOM: There was even a guy whose girlfriend convinced him to stand on the bumper and, uh, contribute to the radiator using nothing but his own devices.

RAY: We told him that it was unlikely he did any damage. But just to be on the safe side he should -- of course -- flush the system immediately.

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