Probably only a mystery to me because of my inexperience buuut....

1) Car was taking a lot of cranking to start, was getting fuel ok, so I checked plugs, only one was functioning.

2) Replaced all four glow plugs with cheap Autolites, there was so much carbon buildup I could barely get the plugs seated to thread them in.

3) Honeycomb was taking a very long time to glow orange still, and even then was not getting nearly as bright as it should. Car was still taking a ton of cranking to get started.

4) I bought a reamer, reamed the glow holes, reinstalled the Autolites. Long glow time/half power persisted, until one time when I was trying to start the car I heard two quiet 'ticks' from the engine, honeycomb flared extremely bright (I instantly reflexively let go of the crank pull), and now the honeycomb won't light at all.

I'm at the end of my understanding of the glow system. All my fuses in the fuse box look good (I don't even have one labeled 'Glow Plugs' anyway, I assume if there is a fuse it is elsewhere). Could it be that the cheapo Autolite plugs were shorted out or otherwise damaged when I tried to use them in the heavily-sooted pre chambers? At this point, I don't know what else to do except buy better quality glow-plugs, install them, hop on one foot in a southward facing circle 720 degrees chanting...).

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1969 220D - 4spd std
1973 220D - 4spd std