I took the hoses off the thermostat housing and filled both the inlet and outlet with gardenhose water until they started overflowing. Next I filled up the radiator hoses as much as I could and put them on. Seems to have fixed my issue.

Still bothering me that I couldn't find the root cause.

Thank you all for the helpful tips!

Root cause was an airlock. These engines are notoriously hard to fill with coolant without causing an airlock. Your airlock was more problematic than most.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

My 616s seem to be harder to burp than the 300TD. What I do is raise the front and disconnect the top radiator hose at the radiator like Kerry said but I also disconnect the heater hose, driver side at the firewall, wire it up and add coolant in the radiator hose till it overflows out the wired up heater hose.

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1985 Euro 240D 5 spd 140K
1979 240D 5 spd, 40K on engine rebuild
1994 Dodge/Cummins, 5 spd, 121K
1964 Allice Chalmers D15 tractor
2014 Kubota L3800 tractor
1964 VW bug

"Lifes too short to drive a boring car"

Now you got to do it again with antifreeze.