Thought it was the O2 sensor, but...(follow up)

[87MJTIM]

I have the REM scanner in my truck.  I had the O2 volt, Open v. Close Loop, ST and LT fuel trims constantly displayed.  The O2 volts was constantly around 12v.  At start up, after approx. 2 min., it would switch to closed loop, then quickly switch back to open.  There it would stay.  Naturally, I thought the O2 sensor was bad/going bad.  I joined in on another thread about O2 sensors.  I had recently replaced it approx 1 ago.  I thought it couldn't be bad, yet.  Based on feed back from Ohm in the thread, I tested the sensors that tell the ECU to switch to closed loop: MAP, MAT, CTS, and the O2.  I changed one of the displays to show Rich/Lean.  It constantly registers Lean, and would not switch to Rich.  I used the Renix Fuel Injection manual and www.lunghd.com for proper testing steps.  All of the readings registered within the normal operating ranges.  (I didn't write them down, but used the scales from the two sources.  SO DON"T ASK ME THE READINGS.)

 

Since all the readings came back "normal," I looked upstream at the ECU.  A few years ago, I had swapped in a 90 ECU.  I decided to re-install the original 87 ECU.  I did this last weekend.  Since then, the REM is showing that it switches to closed loop, the ST fuel trim goes up and down, and the Rich/Lean reading is now switching.  

 

So, to make a short story long, it looks like the ECU was/is bad.

[Ωhm]

32 minutes ago, 87MJTIM said:

I decided to re-install the original 87 ECU.

Good news (lucky you) on having that original ECU. Good call, after proving everything else looked good (love your REM) and swapping ECU's. I'd still be on the lookout now for 89-90 ECU again.

[87MJTIM]

I have (accumulated) 4.  Two 87-88(1 manual [original] and 1 auto) and two 89-90 (1 manual [bad?] and 1 auto).

[fiatslug87]

Good work! I'm still waiting on the OmixAda O2 sensor, should be here tomorrow.