Be sure to track down where the coolant is going. It's gotta be going somewhere if you are loosing alot. Pull a spark plug and see if it's extra cleaned (ie steam cleaned). Maybe do a compression test too. Test or just replace the radiator/pressure cap. If the pressure is too low the water pump won't put out the correct flow (somebody correct me if I'm wrong). I just did a headgasket job on a Honda CRX that heated up real slow in stop and go traffic. The tear was small, but enough to overheat it. The head wasn't warped though, lucky for me. Good luck.

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1994 C 280 117.5k, White (Good as new)
1997 Toyota Camry 149k Miles (Not so pretty anymore)

1990 190e 2.6 95k (Sold-Should not have)
1981 240d Stick ??? Miles...sold