It is freezing in my 1990 Mitsubishi Precis

I am having problems with my heater, 4 different mechanics have checked it out and are as stumped as I am. It only blows out cold air unless you remove the largest heater hose and cap the end with your finger, and only then it works correctly, and believe me everything is working, and been flushed out, checked, replaced etc. and all is circulating well. Can anyone help me figure this one out!!!
Desperate to become warm. :banghead: