(Help) Vauxhall Zafira 1.8 coolant problem

[Johmon]

My friend opened my Zafira's engine coolant reservoir cap while it was running, I just got back from a drive and parked it in the garage but went inside home to get water and my friend said he opened the reservoir cap while it was running and coolant started to flow out with steam he said, when I got back I saw coolant dripping down the floor, he closed it and turned off the engine but when we looked at the coolant bottle all the coolant drained somewhere, I am guessing in the engine block or radiator? but then I started the car and it was running fine but every time the car reached operating temperature which is 90C the coolant rises up and overfills in the reservoir but after it cools down and fans turn off the coolant drains back to wherever, I saw this happen 4 times and I am not sure what to do, I have not driven it since it happend. Can anyone tell me how to solve this issue?

To make it simple my problem is the coolant reservoir bottle is pretty much empty while its running but once the car reaches 90C and the thermostat opens the coolant just rises up very quickly and overfills my tank but after it cools off and fans are off it drains back to wherever and its literally empty, I want to solve this issue.

If you see the picture it overfills to this much.

[Vauxhall Owners Club]

i would suspect the coolant pressure cap as being faulty.

If it cannot hold the pressure beyond boiling point then the seal fails and releases pressure leading to coolant loss and overheating....simple and cheap fix, worth trying first