Coolant Boiling Over After Engine Start

If the radiator boil over after engine start, it indicates a leaking radiator cap or a wrong anti-freeze / water ratio.

Leaking Radiator Cap

The purpose of the radiator cap is to keep the entire engine cooling system under pressure. This has the effect of raising the boiling point of the coolant. If the radiator cap is leaking (i.e. not sealing well), the boiling point of the coolant will be considerably less than normal. As a result, the engine (and therefore the coolant) will only have to be semi-hot for the radiator to boil over.

Wrong Anti-freeze/Water Ratio

As you probably know, anti-freeze is added to water (in the radiator) to keep the water from freezing when the outside temperature is very cold. In addition to this, anti-freeze also raises the boiling point of water. In most cases, a 50/50 mix of anti-freeze and water is adequate to provide both freeze and boil-over protection. However, if the anti-freeze/water ratio is wrong (e.g. not enough anti-freeze and too much water), it'll boil at a lower temperature... specifically, when the engine is only semi-hot.

What to do?

First, when the engine is cold, remove the radiator cap and closely examine it. Look for cracks in the rubber part of the cap's underside. Screw the cap back on, making sure it solidly attaches to the radiator fill hole (i.e. no looseness). 

If anything seems odd about the radiator cap, replace it and see if this corrects the boil-over problem. Otherwise, replace the coolant with a 50/50 mixture of water and anti-freeze. You'll want to first get the entire cooling system backflushed in order to remove all of the old coolants. Why? Because simply opening up the valve at the bottom of the radiator won't completely drain the cooling system.