I need to drive my Integra to a friend's place to work on replacing the radiator. It's been 70 degrees lately, is waiting 1 hour long enough to allow the system to cool down? Any rules of thumb for approximating wait time is appreciated.
You don't say how far the drive to your friends house is, but in my experience an hour of down time results in a pretty big temperature drop. But your milage may vary, so before you proceed check the hoses and radiator to be sure that they are comfortable to touch – the coolant inside will be hotter.
Once you drain the coolant, the radiator will cool more and pretty fast. The big risk is that if you release pressure on the radiator too soon, the coolant will still be above 212° (boiling at normal atmospheric pressure) and will start boiling – and possibly spraying steam and hot coolant with considerable force.
So use your judgement, if the radiator and hoses seem too hot to comfortably touch, give them some more time.
Great question! One hour should be plenty. If you open it too soon, you'll have an explosion of boiling liquid in your face and on your arms. However, if you wait for it to cool for an hour, you should be plenty safe.
Think about it this way: the radiator is designed to take heat off your engine, so it has to be able to cool off in less than 10 hours. I'm exaggerating, but the point is, the radiator fluid isn't in a thermos.
Some forums will suggest only 15 minutes; you're probably okay with that, but you're better to wait 45 minutes to an hour. If you really want to play it good and safe, put on gloves when you pull the cap and turn your face away as you give the final tug.
If you use an InfraRed Camera to shoot the radiator and determine temperature, you'll want the temperature to be below 140 degrees (exposure to 140 degree liquid for six seconds can produce third-degree burns in adults.) Counting in the possibility of there being pockets of hotter steam, I would wait for the temperature level to get down to 130 if your car is in the shade. Obviously, if the ambient temperature is 130, wating for it to get below that isn't reasonable. :)