I feel there is more to this story. If no one wants to work on the car it's generally not the car's fault. From perfect to undiagnosably messed up in a short time is atypical. Good mechanic "actually fixes things" can't fix car??? Also uncommon. Belt and basic maintenance items repair bad trans? Huh??

I have 38 years' car experince and they way this is presented just doesn't work. I can't help but wonder about the good mechanic first. Sorry, but as a mechanic I've worked with too many guys that make convincing arguments that their incorrect diagnosis is right. When they don't fix the problem they are highly skilled at covering the fact with "getting closer" or "made a difference" or "that part was hiding the symptoms from the other broken part." And if good mechanic is actually good then I wonder about "like brand new" status of car.

Why is it rich? What are the symptoms of rich operation? Are you sure the symptom you're seeing is actually causing rich operation? How can O2 sensor be reporting okee-dokee if three injectors are "max rich?" And really, what is "max rich?" In my mind that indicates the injector is at 100% duty cycle... "always on." That's an easily diagnosable problem. The way the problems are reported requires interpretation, which makes an accurate diagnosis tough.

Oscilliscopes were once rare in the auto repair field but a shop without one today is not fully capable of diagnosing problems. The injector on and off time can be measured with an external tool to confirm that ECM is reporting correctly. Time for each injector can be recorded and compared. If all injectors are at similar times then injectors get checked for leaking or improper delivery rate. If injectors are open for same time, and are all delivering same amount of fuel, then it's not injector or fuel delivery problem. Stop chasing injectors and look for other issues. Good mechanic knows when to trust facts.

Spark plugs black? Wet? Dry? Oscilloscope on coil wire will show secondary pattern to determine whether or not plugs are being fired correctly, if voltage is going where it should, if cylinders are rich or lean. Good scope reader can tell if plug gaps are correct, if EGR is active, if cylinder compression is not correct. Good scope reader can see crossfire, failing wires, bridges in coil insulation. Good scope reader can do many amazing things.

How can O2 be "perfect" and engine be "max rich?" Oscilloscope on O2 sensor will show O2 voltage and signal pattern. Does voltage match ecm reported value? Does disconnecting injectors change reading on scope? How about reading from ECM? Good mechanic will be confirming data with trusted tool, not just accepting what's on computer screen.

Is the cam good? Is the vacuum signal strong and steady? A weak cam can produce good compression readings while failing to fill a cylinder fully. Vacuum gauge is one way to see this. Oscilliscope on MAP sensor signal can be even better. Oscilliscope on MAP can be read to see intake valve opening and closing, pressure waves "bounce" as valves are closed, throttle angle changes appear imediately, restricted exhaust shows up quickly. This Oscilliscope thing sounds more versatile all the time.

Why are we not suspecting an ecm? Because someone else's isn't broken? But you want this fixed so how can you rule anything out? ECM response to rich or lean O2 is critical part of feedback loop. High O2 signal voltage from rich engine should result in less fuel delivered. But O2 sensor value is fine: "all sensors are in range and almost dead center of range." So what is telling you engine is rich? BLM high = ecm adding fuel to correct lean problem. BLM low = ecm removing fuel. My feeble mind follows training from 1980's EFI classes. Misfire = fuel not reacting = excess O2 = lean O2 reading = ECM tries to add fuel. Engine is not rich! Engine is misfiring and ecm is responding as programmed. Did Good Mechanic discuss this with you?

I'm not invested in diagnosing this. The questions are for you, not for me. The way this is presented I do not believe there's much chance of successful diagnosis in an internet forum. There are too many subjective descriptions and there's too little actual data to work with. "I changed the injectors but they're still rich" is not the same as looking at a recording of injector on-time over time. "The O2 readings are perfect" does not present an opportunity to evaluate the O2 readings. It tells the reader "Don't bother looking here because you won't find anything."

Hopefully you and your "guy" will find an answer to this obviously frustrating problem.