Are you sure the air filter bracket isn’t cracked? I believe there’s a reinforced version out there, not sure if it’s legit or works. But if the old bracket shook enough it could have cracked.

I thought around 3000 rpm at 70 was roughly right. I should have looked earlier. It’s been a few years since I’ve routinely drove a w123 with a tach.

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Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (113k)
1991 350SD (206k)
1991 300D (228k)
1993 300SD (291k)
1993 300D 2.5T (338k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (265k)

Past Diesels:
1983 300D (228K)
1985 300D (233K)