1979 300D, series preglow system. When cold or very hot, engine starts wonderfully. Once the cold engine has started, even seconds later, the preglow system will not function, for hours. Thus, the hot engine having cooled for an hour or so, will not start without extensive cranking and a big cloud of blue-white smoke. During the times the preglow system is not functioning, the preglow light does not come on, but does when the system is functioning, i.e., starting very cold. Also, during these non-functioning times, it seems to me that some of the system's diagnostics tests per the shop manual cannot be performed.
Questions: 1. Any advice or pointers out there? 2. Where are the Preglow Time Relay and the Temperature Sensor physically located? 3. What is a reliable, least-expensive source for the Relay, the Sensor, and/or Glow Plugs should I need them?
Thank you, thank you, in advance, for assistance.
Bill