I checked the EGR valve per Bentley's manual. (1997 camper)
I do observe the stark change of idle when applying vacuum to the hose.
However, the hose/valve does not "HOLD VACUUM". It "fills" in about 1 second.
Is this expected?
Or am I looking at a cracked diaphragm? (just changed the hose, nothing changed, so not likely to be the hose)
so, I ended up changing the EGR valve. The new one does hold vacuum, so that is the answer to the original post.
It is not hard to swap the valve, but it is a fairly expensive part, I think I found it for about $350 after some searching. (www.RockAuto.com)
The CEL stayed nicely off for a couple of weeks after the change.
I connected a vacuum gauge between the EGR valve and the N18 valve.
It red zero vacuum both idling and revving the engine, even after warming it fully.
That would indicate a stuck-closed N18 or lack of vacuum at the vacuum inlet (well, where the air exits the N18 to go into the engine vacuum ).
Vacuum at the inlet of the valve was about 8(Hgmm?) on my gauge.
I tried tapping the side of the solenoid with the screwdriver handle (old trick with stuck solenoids) and it started chattering/clicking! Then it stopped, then it did it some more...
I still did not observe any vacuum at the EGR.
Then I sprayed some carb cleaner and later some wd40 in the N18 inlets...
probably not a great idea... could affect rubber?
on further tests, It now stopped clicking and it appears stuck open:
I read about 6 (Hgmm) of vacuum at the EGR and it idles very poorly.
Tried tapping some more, tried disconnecting the solenoid electric connector, but it appears stuck open.
At this point I pulled out the N18, placed the airfilter directly onto the main vacuum line, and will look to buy new (or possibly repair/unstuck it?).
Engine light is ON: code 1440, likely because the ECU reads an open circuit in place of the solenoid.