vectra c coolant problem

[Bill,R]

Hi I have a coolant problem on my Vectra c 1.8 petrol.  The coolant thermostat failed so I replaced it and topped up but on reaching temperature it started to blow back out of the coolant tank.  I removed it and placed it in a pan of wate on the stove.  On reaching a rolling boil it failed to open so got it replaced.  It happened again with the new thermostat.  I let the engine cool down and tried again and the same thing happened.  Again the thermostat did not open in boiling water.  Later obtained another new thermostat and tried again.  First time it started passing water through and the radiator was getting hot but there was a lot of banging from a solenoid down on the bottom NS of the radiator. (what is this solenoid for?)  I had to break off then and came back to it later.  Again after reaching temperature it started spewing water out of the coolant tank about over 1 litre.  I checked the thermostat in boing water and again it failed to open even after several minutes.  Any suggestions please.  Every one tells be it is self bleeding.  Thanks,  Bill

[Vauxhall Owners Club]

if the thermostat is failing to open at around 88 degrees centigrade then your supply of stats is not reliable and would shop elsewhere.
Check also the coolant pressure cap as if this fails then it can lead to lower boiling point in the system