First and foremost, if you ever intend to properly learn how to stop a radiator leak, the most and most important piece of information you always need to remember is: DO NOT remove the radiator cap until you are absolutely certain the car has cooled down. Opening a hot radiator cap can blast you with scalding hot liquid that could cause serious burns or even death.
If you can determine the exact spot of your leak, some well-placed epoxy or similar bonding agent will provide a tight, lasting seal that will keep you going for quite a while. For those of you who cannot determine the location of the leak, you are going to have to resort to different measures. On the bright side, the things you need that can help fix a leak in your radiator are, believe it or not, found in your kitchen.
For small holes, pouring seasonings such as ground pepper, paprika or cinnamon into the radiator will provide a nice temporary fix. The seasonings will flow towards the hole, and harden up inside the radiator, thus providing a seal. Here is the real head scratcher: for slightly bigger holes, crack an egg or two into your radiator. The egg will cook inside the hot water, and be forced into the hole by pressure, providing a tight seal.
