What does it mean when your radiator starts steaming while it is running?

If it's the RADIATOR that's steaming it probably means that you have a leaky radiator. If the cap is steaming it might just mean that you need to replace the radiator cap, but it could mean that the engine is overheating. Check the coolant level and make certain that you do not see evidence of leakage anywhere around the engine or radiator. After the engine has cooled, look for light green powder around the edges of the radiator and around the water pump. This is evidence of leakage.