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What would cause AFR to go lean at any throttle input?


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BF XR6 NA with a turbo on it, nearly stalls from leaning out from any throttle input (I've had outright stall twice) slight increase in throttle in idle or cruising does the same thing. tiniest amount of throttle or 'boot it' will have the same results. STFT in gear is within 5%.
If I re-flash the PCM, It will be fine, but it will start to appear after a day or two and gets worse from there.

Once it goes lean the computer riches up the AFR after about 0.5-1 second and its good from there. its worst at stop start driving.

stock xr6t exhaust cat back, 100cell cat, bosch 540 pump, FG Turbo MAP sensor, bosch 0 280 158 040 injectors '1150cc' @ 4 bar, stock xr6t crossover and piping, 105lb valve springs, stock motor from there

Any info would be appreciated, thanks

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brand new pre cat O2 sensor on the dump as the new one was cooked, unsure what the LTFT is at will report back on that, as reflashed this morning, dealing with a ETC fault at the moment 🤦‍♀️


i never got data when i bought the injectors, been dialling them in for a while. stft is pretty good, WOT on open loop is almost dead on compared to commanded lambda. i’m not a tuner and this is my first ever car to go this in depth with. 

barrage tuning had a play with the injectors also.

Edited by JayJay
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when it was NA the O2 sensor was cooked and ran rich all the time, only noticed the 0v output of the cooked one about 2 months ago. 

i’ve got a wideband bunged into the down pipe not wired into the ecu monitoring everything, and an OBD2 HUD showing me trims.

idle in gear, stft is 0-1.5% ltft 0%
cruising -10 to -15% stft -3.1 LTFT

unplugged the O2 sensor, 11-12afr cruising, 15.8afr in gear idle, does spike lean for a split second then immediately goes rich.


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5 hours ago, JayJay said:

it’s only ever started doing this since i’ve put the FG xr6t MAP sensor in and the injectors + reg

Sounds like you have a tuning or mechanical issue then. You are going to need to go back to basic and check fuel pressure, spark gap. Datalog the MAP pressure vs a pressure gauge or check it against the boost sensor etc.

Did you set up the new map sensor with the correct slope and offset in the software?

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22 minutes ago, Roland@pcmtec said:

Did you set up the new map sensor with the correct slope and offset in the software?

I've merged a BF XR6T MAF Map auF0045/6 & Speed density auF0060 auF0053 & auF0057
I've merged FG XR6T Speed density scalars to mine, such as auF0512, auF0511 and left the rest alone due to being the same values.

Data-logging shows fuel pressure at 3.90, I'd imagine that's fine if using a 4 bar reg.
Plugs are just over a year old, but you never know I suppose🤷‍♀️

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6 minutes ago, Roland@pcmtec said:

These cars do not have a fuel pressure sensor. You'll need to check it mechanically. 

I'll look into fuel pressure for sure. prob wont be the worst idea to do a fuel filter while at it, Golebys do a fuel reg + gauge kit thankfully.
The MAF and MAP parameters that I've changed, am I in the ballpark? Thanks.

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  • 3 months later...

yes I'm reviving an old topic, Its been on the dyno, made 263kw and 795nm torque, its been tuned well, but even he couldn't get rid of the lean spot. is there something we are missing? it has a Turbosmart rising rate fuel reg, with gauge, and no drop in fuel pressure, and I threw in a new filter just because, didn't make a difference

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