ECU has a vacuum line, the mixer that was on the truck was in poor condition so I changed it out with a brand new one I had in stock for testing. I will probably re-install the original mixer once this problem is solved. What is very strange is how fast the O2 volts climb to.9. Even if I run the vacuum hose directly to the regulator from the port on the mixer, eliminating the FCSV the O2 volts still peg at.9 There is no doubt the fuel mixture is rich.