Many thanks to Art Bernstein, yama and Chad M for there help and experience with my problem. The solution was replacement of the oxygen sensor. I must have damaged the sensor while replacing the exhaust pipe (between the manifold and the cat). Measuring the old sensor voltage yielded a consistent low voltage that tells the ECU that the mixture is lean, which apparently was not. Measuring the new sensor voltage yields a more typical 0.6/0.7 volt reading. No faults are reported on the blinking LED from the ECU

I also replaced the back pressure controller since I did not like the way the pressure was being controlled. The 5mm vacuum hose and short 8mm fuel line on the discharge side was also replaced. I checked the passage into the intake and it was clear.

Routine replacement of spark plugs was also done since the old ones were heavily sooted.