Today I nearly lost all the engine coolant. I was driving home from the dealer workshop. After checking the cooling system it seems that the problem is the coolant reservoir cap. The O ring in the cap is okay, but some of the plastic surrounding the O ring was missing. Could a leaking cap really blow off such a large amount of cooling water, or is there another problem in the engine.
If the two o-rings on the cap are intact the problem isn't with the cap. Yes, the cap will seep and let vapor out below 20PSI due to o-ring failure, but a failed water pump or cracked coolant spider connection is far more likely. Also take a close look at the tank itself to see if it's cracked. It's plastic and anything made of plastic will get brittle over time.
Today I had the car brought to the dealer. They filled new coolant and a new coolant cap. I drove home and everything seems okay so far. I will look after the temperature and the water pump in the coming days. Thanks for the reply.