I feel your pain. I had a similar issue when I changed my thermostat. It did not leak profusely but dripped a fair amount. I had the o-ring in backwards, so check that. I dont know how to tell which way is right, but no leaks after switching it.

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Treetops
06 E320 CDI 127K Miles
87 300TD 231K Miles

99 E300 269K Miles-Sold