What is it???
 

This little gadget is held to the motor mount bracket on my 1987 190D by a couple of screws. The fuel line from the tank is attached, as is the fuel line to the fuel pump. There are two other fuel lines running to it as well and I can't even find where the go! This gadget is leaking fuel and I'd like to just remove it and plumb the lines together, but I don't know where the two odd ones go. Does anyone know what this thing is for? My 300D doesn't have one. Thanks

I think that is a fuel line heater, it's got a square cut o-ring that starts leaking. about 2.00 fix.
the other two lines are coolant.
call phil, and he will happily send you the o-ring.

That is the fuel thermostat/valve. It diverts the flow of fuel through the heat exchanger loop (metal pipe exiting the rear of the engine with a loop of tubing that has fuel running through it) until the engine reaches a certain temp. I am sure MB felt it was needed when the system was designed, but many people just bypass the heater and thermostat all together. I ran veg oil in my car, so that was one of the first things I removed (my system had plenty of fuel heaters installed, so no need for the factory setup). If you want to bypass it, just run a hose directly from the hard line from the tank to the lift pump (through the inline filter of course).