4 out of 5 glow plugs were difficult to remove! They didn't want to come out even totall unthreaded due to carbon holding it back. What cause carbon to form in the prechamber? It is a dead-headed cavity with no combustion flow through it. You'd think the red hot glow plug would burn whatever carbon gets in there and keep the area clear. Is it due to poor injector spray pattern? injection timing off? Anyway I had to pry the glow plugs with long nose pliers as I turn the glow plug with a gear wrench to get them out. I happen to have a glow plug reamer I made (7 mm dril bit + old glow plug) and it came to good use. It took a long time to ream out all 5 holes as I had to apply forward pressure with one hand while I turn the reamer with a gear wrench with other hand in an awkward position. Wheel bearing grease on the reamer helped it stick to the carbon.

I'll be checking out the glow plug controller next to see if it is misbehaving.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked