Problem solved. The mystery part is actually part of the vacuum controlled 4wd system. I found a vacuum line in the 4wd system that had become disconnected. Connecting it solved what I thought were several separate problems: low power, black smoke, intermittent engine rattle, and no 4wd.
Hmm. Interesting. No possible connection to the OPS? Then I have misidentified something. I'll take a pic. There is some kind of plastic solenoid on the firewall connected to this black ball by a vacuum line. Another vacuum line disappears into the underworld. I dig around and find where it...
I have a 95 6.5l GM Suburban. While digging around in the engine bay I found a vacuum line with one end not connected to anything. The other end is connected to a softball sized black plastic thing mounted on the drivers side firewall. This platic ball is connected to the oil pressure swith via...