RADIATOR BOILS WHEN I SHUT ENGINE OFF

Because the engine is overheating. Overheating can be caused by several issues. Low on coolant, leaks (internal or external), clogged radiator, bad thermostat, cooling fans not running, etc.

Thank you Gary. I drained the radiator and put a new thermostat in. Is it possible to put the thermostat in wrong? my check engine light came on, so I went to auto zone they said plug #4 was bad. So I changed all 4 plugs and put new wires on. my car has 100,000 miles on it. when I took the #4 spark plug out it had oil dripping from it, Is that normal? all the other spark plugs looked good, like a golden brown color. My car still gets hot I think it is the fan. when I have to drive I just have the heater on and try to drive on the freeway until I get the money for a new fan to be installed.

It is possible to put the thermostat in backwards. The spring and temp sensor should be in the engine or block side. If a sparkplug has oil on it, that is not normal. Sounds like you have an internal engine issue like a blown head gasket or something. A compression test is the best way to check for cylinder leak problems.