If I were you I would just put a manual ball valve between the engine and heater core. That way you can use heat and cold until you have time to fix it, but it wouldn't be a bad idea to keep the ball valve in place just in case anyways. I put mine in place years ago after my first heater core failure. I have the ball valve at one end and the monovalve at the other end to completely isolate the heater core incase of another failure.

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1990 190E 3.0L