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1997 Ford F-150 4.2L V6 . Does the OEM ECU use the MAF during open loop and closed loop or just closed loop . Chasing an overly fat fuel issue in closed loop . Replaced ECU , both narrow band O2’s , MAF , plugs . Runs fine in open loop . Gets warm switches to closed loop . ECU starts pulling 25%-30% Fuel . Using HPTuners scanner to see this .

My next step is to repin and New wires ECU to MAF . Plus buy compatible interphase and software . EECV ECU OEM Terry

Have you considered plugged exhaust (catalytic converter)? Is the engine mechanically sound (good compression, no vacuum leaks)? Any chance the incorrect injectors have been fitted? They need to match what the ECU expects.

The MAF should be used in both open and closed loop.

I'm a little confused? Are you saying that in open loop cruise conditions you're achieving stoich (or close to it), but in closed loop the ECU is trimming out 25-30% fuel? That sounds like a narrowband O2 issue to me (although I understand you say you've swapped them out).

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