Trying to fix my Mitsubishi... when I go to WOT (Wide Open Throttle), I pull a PO300 code. No other codes have ever appeared. Only a PO300. The problem is easily reproduced. To fix it, we have tried:
New plugs and wires
Tested for vacuum leaks (smoke test and spraying carb cleaner methods)
Tested fuel pressure (normal)
Replaced MAF sensor

No change in the problem with any of the above. Problem has been happening for 13 months with no change.

I collected some additional data today. I decided to graph the MAF, one O2 sensor, Bank 1 STFT, and Bank 1 LTFT. VERY interesting. When the engine is at WOT (Wide Open Throttle), both fuel trims dive low. In this test, I went to WOT at the hill near my house. You can see the MAF spike when I floored it. But both fuel trims go down!

My research on this seems to indicate that at WOT, the ECU (by design) goes into “open loop” and ignores the O2 sensors, depending on the MAF and ?maybe other factors? to set the trims. It seems reasonable that I had a bad MAF… If the MAF is bad and reading low, then in open loop the ECU would trim down the fuel to match the (erroneously) reported air mass. Since the air mass would be higher than the MAF is reporting , the A/F mixture would be wrong (too much air), and you would lean out, setting the PO300 code. But the MAF is brand new, just installed yesterday.

What am I overlooking here?