Q: why is there air bubbling out of the radiator filler cap on 2004 Volvo V40

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the problem occurs everytime the engine is stated it overheats after about 20 minutes and air pressure is building up in the pipes of the cooling system water is released through the filler cap when it get hot. the temp of the car heater stays cold but the engine overheats
(2) Answers
From cold start with radiator full , watch and see if the 'bubbling is immediate upon starting. If so , have checked for blown head gasket. As for temp. gauge- needs liquid (hot) in order to get proper reading, if system has air in it then reading will be incorrect. Have diagnosed or as already stated in #1 , try thermostat and proper mix of coolant. be sure system is purged/bled properly or more trouble can happen.