Of course, it was not the oxygen sensor. The oxygen sensor was just giving an out-of-range reading, because of another problem. However, I now have a new oxygen sensor, new injector seals, I know that I have no air leaks, I own a Cis-E fuel pressure guage, and I have a much deeper understanding of how the Cis-E system works..

Here are the symptoms:

Cold start is perfect. High EHA current, droping quickly at first, then slowly (as intended), maintaining perfect idle mixture as the engine warms up. It reaches operating temperature and idles well, but the Cis-E signal indicates that it is not in closed loop. All is well.

Then, you touch the throttle. The system goes nuts. The idle oscillates from 800 to 1500 rpm. The air flow meter plate is opening and closing as the amount of air going in changes.

Here is the answer:

It was the potentiometer on the side of the air flow meter. It had a worn spot at idle. As long as nothing moved, it was OK. As soon as the air flow meter opened a bit, it generated erratic signals, and the computer did not know what to make of it.

This is the potentiometer that was available for $50 a few years ago, and is now only available as part of a $1500 assembly. Fortunately, there are auto wreckers.

The old gal now resumes her quest for the 500,000 km grille badge.

Andrew
1989 190e