No pressure in cooling system

[topliner]

Hi, I changed the thermostat on my car yesterday and renewed the coolant, I ran the car up to temprature initially the guage stayed at 80 degrees. I then took the car it for a drive and the guage showed mid way between 80 and 100 degrees, the heater is blowing hot air but what puzzles me is when the car is at normal temprature there is no pressure when undoing the radiator cap. I have since driven 200 miles with no signs of over heating or loss of coolant. The radiator cap was fine before the coolant change and the thermostat is a wahler bought from ECP with the identical mercedes part number on it.
As always I appreciate your help and advice.

 

[Naraic]

Are you actually taking off the cap of the radiator or the expansion tank?

 

[topliner]

Hi Naraic, I'm taking it off the expansion tank.

 

[EmilysDad]

Hi Naraic, I'm taking it off the expansion tank.

I don't know your car, but the expansion tank on some cars (presumably yours is one if them) is just that ...... any surplus coolant escapes into the tank when the engine is hot & as it cools, it then gets sucked back into the engine's coolant system. Others cars, the expansion or header tank is pressurised & part of the coolant system.