replaced a bunch of parts in the cooling system recently and couldn't figure out what these two bolts are for in the thermostat housing. I thought it might be an air bleed but I cracked one open while running and think it had the opposite effect... seems like it just sucked a bunch of air in been searching for awhile and cant find anything. its a 1981 240d

On my 84 300D one was for the EGR Coolant Temp Switch.

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84 300D, 82 Volvo 244Gl Diesel

On my 82' 300D, the bolt on the right is for vacuum lines for the EGR circuit. On the the left, a temperature sensor for the interior blower. It keeps the fan from blowing cold air on the occupants when heat is selected and the coolant is still cold.

Looks like a PO replaced both with some bolts to clean up the engine bay. I also have both ports disconnected, they aren't necessary.

My '81 240D has the same bolts/plugs. I assumed it allowed them to use the same thermostat housing on both 616 and 617 engines. The only wrinkle in my assumption is that a factory rebuilt engine was installed some years before I bought it. Would they have replaced the housing? Would the replacement part be designed for both engines?

No. Maybe.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.