The heater on my '91 318is is not working. I have tried many things but still dont work. New thermostat, new additional thermostat (the one on the heater core hose), bleed the system, etc. I finnaly unhooked the hoses that got into the heater core, upper and lower, and there is water flowing though the lower one, which is the one with the additional thermostat, but no water come out of the heater core, which should be the upper one. Any Ideas?