It sounds as if it is doing just what it should do, but needs adjusting. Duty cycle is tested with engine running. At cold, it will give a fixed reading. As the engine reaches operating temperature, it goes into closed-loop control. At that point, the duty cycle should fluctuate as you observed. However, it should be close to 50%, not 90%. That is adjusted by the allen-head adjuster in the air-flow meter / fuel distributor. Adjust by small increments and wait for it to settle down.

Look at this article in the DIY section of this site, under "Resources":

http://www.peachparts.com/Wikka/EngineControls

Andrew
1989 190e 2.6