To fix a cracked plastic radiator, locate the leak, drain the radiator, weld the crack with a soldering iron, pour new coolant into the tank and test the repair. A soldering iron, a marker, a toothbrush, acetone and a hair dryer are handy in the project.
Let the engine run for 30 minutes, and turn off the engine to let the coolant out through the crack. Designate the crack with a marker. Wait about 15 minutes for the engine to cool down before removing the radiator cap. If the cap is leaking, replace it with a new part according to the user manual. Drain out the coolant if its level is above the crack.
Apply acetone to a toothbrush, and use the brush to clean the crack until the surface becomes dry. Blow the crack with a hair dryer. Scrub the crack with a toothbrush, and dry the area again with the hair dryer. Place a soldering iron over the tie, and press the soldering iron until the crack and its surrounding areas melt.
Dry the area with a hair dryer. If needed, score parts of the tie with a sharp tool. Refill the radiator with fresh coolant. Replace the radiator cap.
Shut on the engine, and wait for about 30 minutes for pressure to build up in the tank. If the radiator still leaks, seek the services of a professional to avoid further damage.