I see a couple of mistakes on this thread.

First, YES a bad glow plug can affect any system, but affects the later systems less. I know, as number five is out, and in the "winter" had to plug in the block heater for a bit. Otherwise she would run on four until enough heat for five to come on.

Second, do not use Bosch glow plugs! They are junk! Went bad in a few days to a month. Got a box of Beru to install, just haven't done it because I need the tool and summer is here.

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Current fleet:

1985 Mercedes-Benz 280TE - Waiting for heart surgery.

1985 Mercedes-Benz 300TDT - Rear ended 23 September 2016 and now looking for a new home.

1979 Mercedes-Benz 300TD - Parted out.

1964 Volkswgen Beetle - Vater's since September 1968 and undergoing a restoration.

1971 Volkswagen Sunroof Squareback with F.I. - in need of full restoration.

1971 Volkswagen Squareback automatic with F.I. - Vacationing with her caretaker until he is in better health.