don't know if they have o-rings where the top and bottom of the body screw together, but I had a set where they had disintegrated. Put them on a bench vice, put new o-rings, and snugged them up and put them back in car. Unless you are hearing the nailing, indicating a bad one, I'd just replace 0-rings, snug up, and replace the heat shields or snap gaskets . .. whatever is called for.

Are you seeing pooling at the base where they screw in? That's what my problem was, in addition to needing return lines, and new o-rings for the primer pump!!!!

If all that fail . . . then look at bringing someone else into it to pressure test them, or replace them.

BTW, what's the full description of your auto? year, model, engine?

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1983 300D, the "Avocado"
1976 240D, 4-spd the "Pumpkin", SOLD to Pierre
1984 190D, 2.2L, 5-spd, my intro to MBZ diesels, crashed into in 2002