I finally got around to order a set of intake manifold gaskets. When elevated the left side manifold (no need to remove the injectors or the fuel line supply) I was delighted (imagine that) to discover that indeed one of the gaskets was not seated correctly. Obviously, there must have been and air leak here for some time now (PO spent a bundle in high-profile p-shops chasing the rough idle and poor low RPM response). Another ‘tell’ was that the cylinder temperature associated with that intake leak tended to run cooler…
The use of engine starting fluid (ether is my choice) idea is a good one (Thanks Steve W!) but it tends to give lots of ‘false positives’ if you are not careful to avoid the volatile vapors reaching the air filter box. This was the lower (cylinder surface) gasket which is harder to reach with starter fluid but it worked well in my hands and gave me reason to “believe”
The take to the garage message is that persistence, aided by smart Pelicans, is at times a better solution than just taking it to the p-Pros.
After all that said…it better idle smooth
Cheers,
Jascha