Tuning Issues & Constant o2 voltage?

[Ninharu]

Ok I'm totally lost on this one...

I just rebuilt my entire engine. I re plumed the entire fuel system from the tank to the returns with stainless 6AN line and a AFPR @ 43psi w/o vacuum. I also put in a set of FIC 650s. Now here is where it gets weird. I found the tuning guide on this site and i have my pocket logger. I set the corrections to a baseline of -17 across the board and started the car. It ran perfect for about 15 seconds then totally started to die. The only way it would run normally was at about -3. Now i know that is super rich and probably no good for the engine. So i started to mess around and trouble shoot. I replaced the post cat 02 sensor and that was half of the problem. Now the first o2 now that is the weird thing. No matter how long i run it and i even tried changing it, the voltage never changes from 1.27 which is super rich and there is no CEL. I can knock down my corrections to the point of stalling and it still doesn't change. And so at idle my STFT is always -16. When i drive i think it reads from the second to balance it out and the second o2 oscillates but under load it is stoich at .96v and my trims balance to 0, but that is at a correction of -3 which i know cant be right. Any suggestions you would have would be awesome.

 

[TotalEclipse112]

On an OBDII vehicle (which you have), the post cat o2 sensor is only used to tell the PCM if the cat is functioning properly, tell it of excess hydrocarbons exiting the tail pipe. It has nothing to do with fuel trims. The o2 sensor right after the turbo is the one that controls fuel trims. From what you are describing that o2 sensor is bad, it should be constantly sweeping from rich to lean, except at WOT or after extended idiling. The voltage that o2 sensor puts out should never exceed .9v and that is super rich, plus your STFT at -16 your PCM is seeing super rich condition and attempting to compensate for it by reducing the injector pulse width. From the mods listed for your vehicle your fuel system is a bit of overkill. My advice for the situation is this first see if there are any exhaust leaks prior to the first o2 sensor, if none are found replace the o2 sensor, if still no results are achived lower the fuel pressure down from 43 psi. If none of those work let me know and I'll try and figure it out for you.

 

[Ninharu]

Thanks for the information... After 2 weeks of messing around with this thing i think i finally figured it out. I decided to run a brand new line directly from the o2 sensor to the ecu... And bingo... Everything leveled itself out. I can set my afc to -18 and its stoich. But still i have no idea why the o2 voltage would not change and was stuck at 1.27v. Now i can tune it no problem and its running way better.