I don't believe the problem is in the ECU. I had exact the same problem with my '89. The reason for running lean can be :

Clogged are broken fuel-injectors

Fuel pressure regulator (pressure to low)

AMM out of spec.

There is a simple (and very cheap) way to make your engine running richer.

Dismount the AMM, take off the inlet screen. Inside the AMM, you see a very thin wire in a smaller tube. This is the measuring wire. Don't touch it !!!. The mass of air that's going to the engine is measured by that wire. If you put some foam between the small tube and the outer tube in the AMM, more air is forced in to the smaller tube, so the AMM gives a higher output to the ECU. You have to experiment a bit how much you gonna clog this outer tube to get the right mixture.

I did this 2 years ago, and the foam is still in there, no more check engine lights since then !!

Good luck !