Are you still getting 2 different set lambda %'s? You stated you read 20% one time then 30% another.
According to 0.73-0121 "Testing electrical components of the KE injection system" p. 17 -- a 20% FOM fault indicates "Full load contact incorrectly connected or faulty. 20% readout only if throttle valve switch (S 29/2) operated."
30% would indicate a problem with your big coolant temperature sensor. It's 4 pin black, at rear of head and gives KE and EZL input into the control unit. 30% could also indicate a short or open circuit in your control unit.
I'm not up to speed on the problems you are having, but if you're experiencing poor fuel economy I would suspect the coolant sensor as the problem, but don't quote me on that since I don't know the circumstances surrounding your problem.
You want to be at pin 3 to check the on-off (lambda) ratio.
Also, if you have a pulse code reader, you can check for fault codes at the diagnostic code box on other side near battery.