Fuel lines is the highest probability on this with the leaking O-rings, as previously noted.

I do not have this problem but am soon to replace mine as soon as I get some more parts in.

Here are some links here that I have found helpful in this endeavor.

$793 fuel leak on E300
99 e300td injector pump leak
Finally - Step By Step Glowplug Change - '98-'99 E300!

There are more, do a search.

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raMBow

1999 E300DT Obsydian Black Metallic, Heated Full Leather Parchment options, E2, K2, 136,000+, best 36.5 mpg - GP's 12-04 & 11-12 Zero Stuck
2010 Honda Odyssey - The BrideMobile - best 26.5
(2) 2005 Honday Accord- (1 -Corporate 1 - Personal) - 110,000 4-cyl 30mpg
2000 VW Golf GLS TDI, Upsolute Chip (sold to Brother, now 300+k on it) 48.5 mpg like clock work
1987 Honda CRX HF - Sold 87,000 always over 50 mpg Max 67 mpg