So did you have to take the whole unit off and unhook all of the fuel lines, or just the one on the end?

__________________
1996 E300 D
1992 300D
1990 Ford E-350 7.3 Diesel AKA "the Deep Fryer"

As someone commented it is the T-stat/distribution valve for the fuel heater. I took the whole unit off (2 bolts plus fuel lines - 5 minutes) for ease of access (it is under intake manifold and difficult to get at on my 190D). Once on the bench it took longer to clean it up than it did to change the o-ring.

Jim

__________________
2005 C240 4matic wagon (daily driver)
87 190D - 225K (on loan)
85 190D - 312K (on loan)
2011 Subaru Legacy AWD (Wife's)