ok two types of engines fitted to these astras .igm and izuzu ,both with similar cooling systems .replace thermostat ,may be closed.before suspecting head gasket drain system inspect top and bottom hoses ,ensure cooling fan is operating .refill system with heater vented and on full blast ,engine running, check for signs of oil in coolent if there is then its head gasket failure. hope this helps good luck.
killerdanny, October 2011
