Every one of the adapters in mine has one. This was why I suggested just robbing one out of a different fitting if the one supplied is no good. Maybe the quality control is so bad they let some out without valves?

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]

Well that explains how others were able to get a reading with the HF unit. Mine not only does not have any valves, the fittings were never even threaded to take them. That led me to believe they were just junk.

I have a MAC tools tester now anyway. It only comes with the pencil and loop glow plug fittings for the Mercedes. No injector fitting but it works well.

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1984 300D Turbo 200K

You're better off with the glow plug adapter. You don't have to fuss with the return lines using it. It can be a pain with carbon-ed up plugs though.

It's always something.

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“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now