OK, I went reading other posts on "fuel return check valve" and I also found references to poor idle, which I also have.

Where exactly is this valve? I kinda think some of the pictures were showing a banjo bolt on the engine block side of the IP pump? Just take this off, clean it out, stretch the spring to 27mm and replace? (If I have the type that can be serviced).

Could this be causing the primer pump to not prime the lines?

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Mike Frederick
1986 300SDL, 240K+ miles
1985 300D KaliKar, 270K+ miles