When you have a radiator cap that won't seal properly, you've got a major problem on your hands. Not only does this cause your car to overheat, it can also lead to much larger problems down the line. Some of the signs that you have a bad seal on your radiator include puddles of coolant forming beneath your car, unexpectedly low coolant levels, and overheating problems. The good news is that a fix is easy and cheap. The bad news is that that fix is most likely temporary, and you will eventually have to replace the radiator.
Locate your radiator and find the leak. Once you are sure you've got a leaky radiator, let the engine cool off before proceeding. Be aware that there still might be some steam and pressure in the radiator, and take precautions. There should be a pressure release valve on the radiator itself. If so, depress the release valve to let off the steam.
Open the radiator cap. Add one to two tablespoons of coarse black pepper. Fill the radiator to a proper level with water, replace the cap, and go for a 10-minute drive. The pepper should mix with the water and expand, filling up any holes and stopping the leak.
Park and let the engine cool off again. Once the engine is cool, check the coolant levels and look for any puddling in or underneath the radiator. If the leak hasn't stopped, repeat Steps 2 and 3 until it does.
If pepper doesn't do the trick, go to your local auto supply store and purchase a car radiator leak sealant. Some radiators are made of aluminium, while others are made of copper. There are different sealant products for each, so be sure to purchase the correct sealant product. Follow the directions on the package.
Radiators don't last forever. If you have a leak, you can battle against it for awhile, but you'll eventually need to replace the radiator.