If the radiator cap does not fit properly on the thermostat housing will that cause pressure buildup.?

changed thermostat housing and radiator cap recently and finding that pressure is being released and water is bubbling in resevoir. one mechanic is saying head gasket and other mechanic says thermostat housing is not the right part causing pressure to escape from radiator cap. which mechanic is right?|||The reason why water does not boil in a radiator is the pressure kept by the system. IF the cap leaks there is no pressure allowing the water to boil. The easiest way to fix that is get the right cap and housing that do not leak. That being said aa blown head gasket can allow exhaust into the cooling system causing similar problem however this is a much more expensive fix.I would start with the cap.If the cap doesnt fix the problem have the mechanic use a device that %26quot;sniffs%26quot; for exhaust gas around the radiator cap with it off and the engine running.Be there when this is done so you know for sure.|||both previous answers r right on,|||Could be something simple like air in rad. If it was head gasket it would volcano out of reservoir after revving the engine|||OK is the Housing you Replaced the Correct Part Number for that Engine? With the Radiator cap off and the Engine, do you Smell Exhaust in the Radiator (Head Gasket)? Why did you Replace the Housing %26amp; is the %26quot;New Thermostat%26quot; Installed Correctly? is you %26quot;Coolant Mixture (50-50 Clean Water %26amp; Anti-Freeze used year round) Correct?