I have a '97 5.0 out of an explorer (gt40p I believe) in my Ranger, with a small remote turbo @ 5psi, 255lph walbro hi-pressure, 39# injectors, AEM air fuel gauge (wideband), 90mm lightning MAF. PCM tuned by Sniper Special Forces (which I no longer have acces to).

A/F ratio was tuned to richen down to 11.7 (actual 11.6-11.9 on a/f gauge) when you get heavy into boost. All of the sudden it no longer does that and remains the constant 14.7 (14.6-15.1) even into boost which is dangerous as I understand it. What could cause this? I'm told the PCM can't 'lose its tune' (but it sure seems like it), so what mechanical mechanism can be fooling the computer into not richening the mixture?

It also has a miss (stutter) while at cruise speed (about 2000rpm/65mph on the highway). So far I've changed out a fuel pump, new plugs, fixed a broken EGR rubber hose (blowtorched my heater box). Have a new plug wires and DPFE on the way. Idles great, runs great under any kind of minor (or even major) acceleration.

I'm at a complete loss, and I'm skeptical my local (and only near) dealer will be able to get a grip on this custom application. Any advice would be greatly appreciated.