I recall reading that 1985 300D engines have the gold rack damper bolt. I moved the one from my failed 1985 engine to the earlier replacement engine, but didn't notice a difference in idle at any adjustment. Took another one off a junkyard 1985 for my 1984 and same "no help" result. Thus, try it but don't expect a miracle.

A good diagnostic would be to measure pressure at the IP inlet. I recall some have drilled & tapped a hole in the banjo fitting to install a tube to a pressure gage. Should be >10 psig to not starve the IP. I think the regulator is a simple spring-loaded relief valve which vents excess fluid into the tank return tubing. In engineering world, such is termed "back-pressure regulator".

__________________
1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans