If you can mechanically open the valve with the engine running it should stumble with the big vacuum leak created. There is a tube from the valve around the back of the head to the bottom of the intake that clogs, if no stumbling that tube is clogged. Replace it with new if possible first, second is to try to clean it out with an old speedometer cable...not fun.

Good luck!!!

__________________
"Rudeness is a weak man's imitation of strength" - Eric Hoffer

Would seafoam help to unclog it, if it's clogged?

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1979 Black on Black, 300CD (sold), 1990 Black 300SE, Silver 1989 Volvo 780, 1988 300CE (vanished by the hands of a girlfriend), 1992 300CE (Rescue).

If you could get it directly into the tube, maybe. Some have disconnected the tube at both ends and worked old speedo cables through them. One end is at the valve, the other is under the intake manifold near the center below the throttle plate.

From what I scan tell the tubes are NLA.

__________________
"Rudeness is a weak man's imitation of strength" - Eric Hoffer

The EGR valve is NLA as well. What I've read it ain't easy to remove the pipe underneath the intake manifold without removing it first.

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1979 Black on Black, 300CD (sold), 1990 Black 300SE, Silver 1989 Volvo 780, 1988 300CE (vanished by the hands of a girlfriend), 1992 300CE (Rescue).

Couple of thoughts...

Perhaps a used pipe from a forum member or a junkyard would allow you to thoroughly clean it before install and study/plan on removal and install.

IIRC the connection at the bottom of the intake has a fitting between the pipe and the intake. It may be doable to remove that fitting to gain access to the inside of the pipe to clean it internally without completely removing the pipe.

Start soaking both ends of the pipe at the fittings with penetrating oil (not WD-40). Do it multiple times.

Good luck!!!

__________________
"Rudeness is a weak man's imitation of strength" - Eric Hoffer