I still have all the original glowplugs with no probs at 43k, knock on burlwood. My mom has a '99 E300 as well, and at 37k she had one replaced. The are supposed to do about 100k, although others have had to replace them more frequently. The injectors are the same way. If you use poor quality fuel they won't last long.

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2004 Ram 2500 Cummins HO
2000 Jetta TDI
1999 E300 (sold)