overheating?

I HAVE A CITROEN ZX 1.9DSX ON A P PLATE IT IS USING ALOT OF WATER THE HEATERS BLOW HOT AND COLD ON THE HOT SETTING, IT IS NOT BOILING THE WATER AWAY BUT THERE IS PRESSURE PUSHING THE WATER OUT OF THE OVERFLOW PIPE ON TOP OF THE RADIATER. THE WATER IS A BROWN COLOUR ANY INFORMATION WOULD BE MUCH APPRECIATED THANKS