Newbie fuel trim questions

Well, I'm trying to learn about all the stuff I need to know so that when I get an SAFC I will know how to tune it properly. I've already got a pocketlogger and I used it for the first time today. I check my CEL and it had a code of PO170, I looked it up and I remember it said something about fuel trim but I forgot exactly what it was. What does this code mean? A bigger question....what the hell is fuel trim? On the logger it shows fuel trim 1 and 2. I just want to know what all this fuel trim crap means and how will I know how to adjust it.

As you know I am turboing my 2.4 Spyder. Here is something about tuning with the SAFC that a guy wrote that turboed his 2.4. And I don't really understand what he was saying.

CAUTION: If you kept the stock Spyder ECU, It is very important that you keep a POSITIVE STFT and LTFT (Short Term Fuel Trim and Long Term Fuel Trim). The 4G64 Spyder ECU is somewhat unique in this matter. If you have a NEGATIVE fuel trim (even STFT of -5%, with LTFT at ZERO), the ECU will lean out your Air/Fuel mixture badly at WOT (wide open throttle). I'm sure the Mitsubishi engineers did this for emissions reasons, but it caused all kinds of problems when I first started tuning my car. No matter what I did to the SAFC (even added +50% air), I could not get my engine to run the proper rich A/F ratio. Thus, I leaned out the car furthur, so that in closed loop operation it was adding fuel to compensate (i.e. POSITIVE STFT AND LTFT), and all my tuning problems were fixed. This is why you MUST have an OBDII datalogger. It's the only way to see the STFT and LTFT readings.

Can anyone help me out? I asked my LS1 buddy about fuel trims and he didn't know either. So I figured if anyone would know than it would be the fellow DSMrs.

Also about o2 voltage. About what voltage is ideal and does the a/f ratio go lean or rich when the o2 voltage goes up or down?

Thanks for any help in advance. Sorry about all the wierd questions.

I don't know too much about that but I notice he says if you keep the stock spyder ecu... Maybe you should switch it out for a turbo 2g ecu, would that work?

I don't know too much about that but I notice he says if you keep the stock spyder ecu... Maybe you should switch it out for a turbo 2g ecu, would that work?

Yeah, ECU swap would work but it would be a huge pain in the arse. I'm sticking with the Spyder ECU + SAFC 2 combo.

I'm not sure what exactly you are asking about the fuel trims. So I'll give a basic explanation.

Fuel trims are a way for the ECU to apply a "correction" factor to its fuel maps, based on feedback it gets from the front O2 sensor. There is a short term trim, which is what the ECU is doing right now to get the target AFR at the front O2. This value is slowly over time dumped into a long term trim, to keep the short term trim from maxing out at either end of it's range. For example, if STFT tends to hang out around -8% to -10% for more than 10 minutes or so, the LTFT will move to about -9% and STFT will go back to around zero. LTFT is something like a decaying average of STFT value, if that makes any sense. :)

Negative trims indicate the ECU is pulling fuel OUT, so this indicates a rich condition. Positive trims are the ECU Adding fuel, indicating a lean condition. Note however that once the trims stop adjusting and LTFT becomes stable and STFT is not maxed out, there is no longer any rich/lean condition. It has been compensated for and the ECU is satisfied with it's AFR at the front O2 sensor. The whole point of trying to get trims to zero, or 100 in 1Gs, is to get them at the center of thier adjustment range. This way the ECU can make changes in either direction up to it's full capacity. If you are constantly cruisng around with trims nearly maxed out in one direction, and a weather change or something makes the ECU try to adjust more and there is no room left to adjust, AFR will no longer be what the ECU wants.

The O2 sensor is a narrowband sensor, 0-1 volt. It is only accurate just under 15:1 AFR, which represents about .5 volts. Anything above over below this is not an accurate indicator of AFR, which people have been saying for years regarding WOT tuning. THe ECU itself from the factory also has no faith in anything but the .5 volt reading. The ECU will cycle the injectors so AFR will cycle up and down from about .2 volts to about .8 volts in closed loop operation. The ECU will simply make adjustments so the cycling is centered directly around .5 volts :) SO the AFR is not always ideal, but on average it will be around 14.7:1. New SULEV vehicles that need to get emissions very low run true wideband sensors so the ECU can actually maintain any AFR it wants to in real time. I'd be surprised if they still used the shyte cycling method on those cars.

A lower voltage is leaner, and a higher voltage is richer. If the sensor starts to drift, the ECU still relies on it for AFR feedback, so your true AFR can suffer. This is why a failing O2 sensor (almost always fail low, or lean) causes the ECU to dump in more fuel and run the car too rich killing your fuel economy. With a good sensor the ECU does a very good job of AFR control in closed loop operation.

On the turbo cars at least, trims are completely ignored at WOT. I don't know anything about all that spyder crap above, you're on your own with that :D Hopefully this very basic explanation gives just enough info to help you understand what you need to do without overcomplicating things.

Yeah that pretty much sums it up. You answered all my questions. Thanks.

vanilla gorilla