could just be a gummed up IP from the WVO. i've heard people using Acetone to soak the inside of the IP and it will clean it pretty good (Acetone is very aggressive though , so be aware of that)

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1983 300SD (daily driver)
1991 420SEL (work in progress)
1979 300SD (future replacement for 83 SD)

RIP:
1983 300SD (totalled)
1986 420SEL