This.

Try this first.

RwP

 

I don't want to hijack the thread. I'll start another " wrong ECU injectors = low MPG?"
The O2 sensor is 3 years old.

 

This isn't directly relevant to your situation, but it's useful information that someone might easily find in this thread.

I needed to replace the ECU in my '91 TBI 5.2 a few years back and all I could find at the wrecker was one from a V6, which (obviously) wouldn't work as-is. Here's my blog post about it:

https://nitpickingbastard.wordpress....il-of-the-mcu/

It gets into the computer-geek weeds a bit, so you can scroll to the bottom for the short strokes. Don't try this at home if you don't have experience reworking through-hole circuit boards - you can't afford to damage either the board or the ROM. It's not too hard to find someone with the tools and skills.