In one of my 82 300DTs the heater isn't working. I have replaced the brushes on the blower, the climate control unit, the monovalve, thermostat, radiator, and water pump all to no avail. Where should I turn next? The a/c works but the heat doesn't. I haven't noticed any leaks from the heater core nor am I loosing a discernible amount of coolant. When the a/c comes on we don't smell anything weird but when we switch it over to heat, the heater never comes on. Something else I have noticed (that might or might not be related) is that the coolant doesn't seem to cycle as thoroughly as it should. I know this because I tapped into the cooling system on both of my 123's (right at the oil filter in order to heat veggie oil) and this one's hoses don't seem to get as warm as the other one. This is causing veggie burning issues and possibly has led to coked injectors.
Any suggestions or advice would be of great service.
Thanks,
Julius