Back firing through the intake

What causes a 1992 Lincoln towncar signature series with a 4.6 engine to back fire through the intake?The car will not drive when it is cold you have to let it warm about 10 min. On normal days more on colder days.I will run decent after you let it warm up. After letting it set for an hour you have to go through the process again. The car will idle ok I took the exhaust loose from the manifold but no difference. Recently changed temperature sending unit guage now works. Plugs look good no oil fouling no carbon build up.