A little more on the idle and throttle body...

Plugging the idle air circuit is not a substitute for using the idle test ground. Bosch would hardly have gone through the effort of creating that test point if a sandwich bag would have done as well.

The sandwich bag is for you to chase leaks of air through the throttle body and beyond. This trick is merely to completely close off the idle adjustment path, because grounding the test point does NOT attempt to do that.

Grounding the test point merely fixes the idle air valve's duty cycle (it alternates between closed and open 100 times a second -- that's how it works) so that the baseline idle bypass air can be set. With the test point ground removed, and normal operation restored, the air that the valve allows to pass must be able to be adjusted, by that throbbing valve, based on the computer's reading of rpm and other factors, above and below that base idle setting.

If your Problem Solver book says otherwise, toss it out. This seems to be another common misunderstanding Volvo owners share about "base idle" and the correct adjustment of throttle plate or bypass screw. Maybe PS is responsible?
--
Art Benstein near Baltimore

Never put both feet in your mouth at the same time, because then you won't have a leg to stand on.