water steams but engine not overheating?

When COLD - check the cooling system has enough coolant in it.
Check the engine cooling fan is working correctly - i.e. the fan comes on if the engine is left running for a while when stationary. It's possible that the engine will only overheat if it doesn't get enough air flow from driving if the fan doesn't work - therefore, most of the time it's fine.
Check the temp probe for the gauge.
Check the cap for the overflow tank - it's designed to blow at a set pressure - it could be that it's blowing too early.

DaveB
July 2007