Originally Posted by DieselAddict

I will have to dissent here and personally I'd recommend Bar's Leaks Radiator Stop Leak (I think that's what it's called). However I'd also recommend only running it through the heater core and nothing else. The way you do this is by using an additional aux. water pump (the electric kind, get one from a junkyard) and temporarily connecting some hoses to circulate the Stop Leak / hot water mix only through the core for at least 15 minutes, then flush it with clean water. It worked in my 240D and fixed the problem completely (system maintained perfect pressure afterward) and the heater continued to work just as good as before. I got this recipe from LarryBible. Do a search and you might be able to find his original post with more detail. He's stated that even after 10 years this "fix" was still holding up well.