my 1996 vauxhall astra overheated today, what could cause this?

Question

oil is fine, radiator ok but it over heated.water tank leading to radiator was boiling as was temperature gauge.
filled it with water again once cooled but ran it for 5 minutes and temp boiling again.

Answer 1

Bad Radiator (does not transfer heat efficiently)
Bad Thermostat (will not open correctly)
Bad Water Pump
Bad Fan Belt
Also could be old coolant

You can brush the radiator with your fingers if the fins or metal fall off easily you will need to replace your radiator. If it is the thermostat you can easily by removing the thermostat and placing it in a pan of water and bring it to a boil. If you see the thermostat open you know it works but this is a lot of trouble to not replace it anyways as the only cost 10 to 15 dollars.

The pump would be the hardest to replace and the coolant can be replaces at any service station.

Also pure water is not the best thing to use. You should mix it (depending on your climate) with glycol (antifreeze) as this will 1. keep your radiator from corroding from the inside, 2. raise the boiling temperature, and 3. keep the water from freezing and cracking your engine block

Answer 2

Did the belt come off of the water pump ?