What causes Glow Plug failure?
 

My 71 220D has had an issue for a long time. When I had the original loop style it would often burn out GP's on longer drives (Over 70 miles). Years ago I upgraded to the quick start pencil plug and now the #1 plug went out and the other 3 are fine. I am just curious what keeps causing this. Any hints???

Bosch In Deutschland ? (So they can sell you New Ones!)
 

Something made of Metal that GLOWs 1200 degrees F,Multiple times AND it
Goes Bad?

It does not sound like your pencil plugs failed prematurely. The old loop style on a 220 should last for a decade or so though unless there is somethig unusual going on. Were they Bosch plugs?

Wrong spray pattern from an injector can cause repeat issues. Generally though I have just thought of glow plugs as light bulbs.

Many reasons for them to drop out unexpectably both self inflicted by quality issues and externally by the severe operational conditions.

Would not that depend on the exact meaning of "years ago?"

Was it three years or thirty years?

X2 on the Injector Spray Pattern issue on the Filament/Loop type Glow Plugs.
Pic of what is in the Factory manual.

More normal issues are simply how you drive (does the type of driving you do build up more Carbon; like only low Speed City driving).

How, often do you need to use the Glow Plugs. A recently rebuilt Engine may not need much Glow plug use to get it going. A worn Engine may need several applictions of the Glow Plugs to get it started.

It was about 6 years ago.

Why do they fail?

The FSM makes reference to the length of time a glow plug is used. The newer W123 / W126 glow plug systems have a safety cut out facility to stop them from being on for too long.

The FSM says that carbon build up between their ends and the precombustion chamber can cause premature failure ( shorting out ) and suggest reaming it out as periodic maintenance. Many threads on how to do that properly with heavy grease to catch and remove the grease.... with warnings not to use the engine turning over or compressed air as that may send that carbon the wrong direction .. meaning stopping up the 7 tiny holes in the end of the precombustion chamber critical to fuel atomization...
The newer pencil type plugs can be glowed for 60 seconds straight according to the book....and should be at least 45 seconds if some factors are causing hard starting... like cold weather or lack of compression....or lack of starter speed...