LaXGSR

Hey all, I have a completely stock X, down to the paper air filter. In the winter, we get 91 octane with up to 10% ethanol in it. I did some logging and noticed LTFT Idle = -4.7, but LTFT Cruise = +3.7. Why would one be negative and the other positive? Do I need to scale my maf or injectors (I'm running stock scalings now on a tuned map)? It just seems odd to be that a stock car with stock scaling would be off by over +- 4%.

Brokenevo

iTrader: (18)

thats an interesting thing! Will be interesting to see whats up!

Clipse3GT

iTrader: (10)

I really wouldn't worry about it as long as they stay +/- 5%.

GST Motorsports

iTrader: (8)

This is what fuel trims are for. Different fuel mixtures, temp, altitude, etc

Perfectly normal

- Bryan

Hiboost

iTrader: (9)

You could try going richer (lower the injector scaling) by one notch and see where it settles after a battery reset. This would get your Cruise LTFT closer to zero which is the important one since it adds that value to your fuel delivery at WOT unless you have disabled that. It will also richen everything across the board somewhat although if the Cruise LTFT moves closer to zero it will add less fuel at WOT as well to counter that. The idle LTFT can be off quite a bit and you really won't notice any change in the way the car drives. How is the car tuned otherwise afr wise? I'm assuming you didn't tune with a wideband yet and just got a rough tune or was it on a dyno?

LaXGSR

Thanks for the feedback guys! What threw me off is why idle is negative, but cruise is positive. In the summer, cruise LTFT usually is around 2% or so, but I've seen it as high as 4.8% in the winter.

The car is tuned to settle down to about 11:1 AFR after full boost is reached (wideband verified) on both the street and dyno. However, the actual AFR seems to fluctuate from 11.3-10.7, most likely due to LTFT affects. I haven't disabled LTFT from WOT since I think it's a good safety buffer for different fuels etc. I might try lowering injector scaling slightly (how much?) since I've never seem LTFT Cruise go negative. Maybe I can get it slightly closer to 0 in the summer, and +2 in the winter, so WOT won't be affected as much

LaXGSR

Update: Without having a chance to change anything in my tune, my LTFT cruise is now 6.25%! Idle is 0.19%. What's up with that? Only changes in the car have been new OEM air filter drop-in, and cleaned the MAF with CRC Maf cleaner. If the MAF were bad, it would read odd airflow and load values right? Those still look ok. Any thoughts? Thanks!

SiC

iTrader: (1)

Trims will drift around with different ambient temps and actually with your driving style too. Since it's getting warmer, you'll probably notice more changes which is completely normal

LaXGSR

Thanks, I just thought the goal was within +- 5%, and yet on my completely stock car (except for tune), it's at 6.25%, so I was alarmed. I've never seen it so high before...

fostytou

iTrader: (15)

No reason to be alarmed. 5% is a target, not an absolute.

LaXGSR

Thanks for the reassurance! Another related question - last night after some data logging, I was stopped and idling, and decided to check STFT. It read 0 and stayed there. I then hit the gas and it bounced around betwee +-1% and then seemed to settle on 0 again. Is that normal? I don't think I remember seeing it stay at a 0% STFT before. Thanks!

SiC

iTrader: (1)

Actually 0% STFT is ideal, it means that your idle AFR is settled at 14.7 with your current Idle LTFT. So this means your ECU doesn't have to make any minor corrections to your fuel trims which is good

LaXGSR

Thanks I know 0 is ideal, but I was more concerned that my o2 sensor is going bad or something I don't think I've seen it actually stay at 0% like that before...I guess the o2 sensor must be working since if I blip the throttle, it will start to fluctuate away from 0...

Hiboost

iTrader: (9)

I've had the LTFT swing as much as 5% in one drive cycle, which is why I disabled that from effecting WOT operation as it drove me (literally) crazy! I could go WOT just after the car warmed up getting on the expressway and look out my rear view and see barely any smoke and running high 11's AFR, then 20 minutes later it would be running low 11's AFR and it was like James Bond with the car a notch slower. I caught it on a datalog a few times and literally that 5% change can easily change AFR's by .5 or more.

Having close to zero STFT's means the LTFT's have settled more or less on where they need to be so that the ECU doesn't need to make much of a fine tuning adjustment to reach the 14.7 AFR's during closed loop operation. If it is consistantly hanging around the same 5-6% LTFT then the tune should be pretty stable, you'll find with more mods that it gets rather unstable at times. Keep in mind that if you were to disable LTFT's from effecting WOT that your AFR's would get about 6% leaner so plan accordingly. Worst case you can richen up all your tables a few notches and then work your way leaner again to do it safely.

Golden

iTrader: (1)

My only issue with disabling LTFT affecting WOT is what if you got some bad gas, had a change in ethanol content (+/-10% E), or your fuel pump all the sudden had issues and was pumping less. All of those can change your LTFT a bunch and can save your *** when you go WOT.

But I understand your issues. I feel like my car resets itself after sitting over night to 0% LTFT and after driving it for a while, goes back to it's normal 3.66%.

I wish there was a Long Long Term Fuel Trim (LLTFT) that could be used for WOT and would be more constant.