I've noticed some moisture down there as well. I intend to spray the whole area down with brake parts cleaner before diving in. This will isolate components that are just getting the runoff but it should reveal the real source of the problem.

I am inclined to believe that the hoses should be replaced first before digging into the pre-heater. My 92 looks to have original hoses and those degrade over time due to fuel exposure.

steve a

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Steve A
92 W140 OM603
97 VW Jetta TDI
90 Passat variant TDI 6 speed MT
94 Chevy K1500 6.5TD
05 E320 CDI
+ others