What you need:
- Plastic wrap
- Rubber band
- Place plastic wrap over top of radiator cap opening. Push in slightly, leave a 1-2 inch “U” shape in the plastic.
- Wrap rubber band around the plastic and seal it to the radiator neck. Pinch the overflow hose to prevent air leaks.
- Disable the ignition system or have someone start the engine for a few seconds.
- If the slack you pushed into the plastic wrap pops up, then you more likely have a blown head gasket or cracked head.