I'm having the same problems. I bought a couple of bottles of cooling system flush and now that there's no danger of freezing, I'm going to pour it in and let it run through for a couple of days. I also have a mysterious leak right beneath my upper radiator hose, possibly a cracked neck thing, that's keeping me from putting my cap on all the way and presurizing the system.

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Seth

1984 300D 225K
1985 300D Donor body
1985 300D Turbo 165K. Totaled. Donor Engine. It runs!!!
1980 300SD 311K My New Baby.
1979 BMW 633csi 62K+++? Dead odo