This is the outlet hose from the heater core connector on the firewall on all E30 M3 models..
This part is listed by BMW as BMW part number 64211381541 (64-21-1-381-541) and is described as OEM Coolant Return Hose for E30 M3 (1988, 1989, 1990, 1991).
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BMW Fitments:
3 Series
E30 (84-91) > M3