It's a long shot but I was just working on my timing chain and while trying to rotate the crank I ran into the same problem- a hard stop. After monkeying around with the cam/valves, chain, pump, etc. I found that the assembly that holds the engine struts on the front of the engine had rotated down and was contacting the crank pulley. The little stud on the crank pulley ran into it and was hung up. All I had to do was get the stupid engine strut assembly out of the way, took 2 seconds and zero tools. You might not be as lucky but check the crank pulley just to make sure it didn't run into anything.

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1987 300SDL 167k
1992 Volvo 740 140k
1990 Volvo 740 250k
1989 Volvo 240 269k

Anyone want to trade an old Volvo for an '87 300sdl?