All incorrect so far. This is the fuel thermostat, and it controls flow of fuel to/from the fuel heater (which uses coolant to pre-heat the fuel).

It has one o-ring and an internal copper washer that are replacable. Usually only the o-ring needs to be replaced to fix any leaking.

__________________
Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC