EGR System Working Again!
After spending two weeks
contending with the fuel-mixture codes (which turned out to be caused by a failing MAF sensor) I retraced my steps troubleshooting my EGR problem. Long story short: It was caused by a vacuum leak in the connection from the solenoid vacuum controller to the EGR valve. Specifically, the rubber elbow connector at the EGR valve.
This leak was hard to find and had the side effect of simulating a failure of the solenoid vacuum controller. That is to say, when I got to the troubleshooting step suggested by @ZVOLV, where one electrically actuates the solenoid, there was no change in engine idle speed. So I concluded the solenoid was bad.
I went to the salvage yard where there happened to be a California-equipped 740 with the EGR system. I pulled its solenoid vacuum controller, and while doing so, I also pulled off all four of those rubber elbow connectors, thinking they might be in better condition than the ones on my car. When installing them on my car, the last one I replaced was the hardest one to access -- the one on the EGR valve. Only when I was holding it up close did I notice a small tear in the molded corner of the elbow. Totally invisible when the elbow was in place, obscured by all the other plumbing under the intake manifold.
I would post photos of this but I don't have a hosting site. I'd be happy to send photos in response to a PM.
I have a theory, along the lines that @bobxyz suggested, that this tear was latent and harmless much or most of the time, but could have opened up slightly under acceleration when the engine leaned over a bit in its mounts and stretched the plastic vacuum line running from the solenoid to the valve. That might explain why I was getting those codes intermittently, mostly when accelerating.
In the end I left my original solenoid controller in place -- there's nothing wrong with it -- and I'll take the other one back to the salvage yard for store credit. Although they're no longer available as a new part from Volvo, salvaged ones are in plentiful supply on the auction site.
Finding a source for those rubber elbow connectors is a different story. They have different inside diameters at each end -- smaller for the tubing, larger for the spigots on the EGR valve and the solenoid controller. Very specific to this application, and apparently no longer manufactured. I will experiment to see if my torn one can be re-sealed with a product like Shoe Goo.
It's scary to realize that someone's car might fail its smog test, and possibly be condemned to the crusher, because of a concealed crack in an aging rubber part.