Blocked Heater core hoses... causes over heating?

I am working on a friends Neon and he been having an issue with constant overheating even through he has replaced the thermostat, the radiator, water pump and related hoses. However, he is missing the cap to the coolant tank and his heater core hoses are blocked, both incoming and outgoing.

Could the blocked hoses be causes it to over heat?

Are you sure its overheating and not a temp gauge, sensor issue? Has the cap been replaced?