Fuel trims---
Just by looking at your LTFT's - Long Term Fuel Trim, you're not going to be able to tell if your car is running rich or lean. Its just telling you how much % the PCM is compensating - or + so that you DONT run rich or lean.
The PCM comes up with a value for how much fuel to inject, and uses exactly this value when you first reset your PCM. It then learns: it counts the number high and low pulses from your front O2-- anything over 450mV is considered high and anything under is considered low (that is all that narrowband O2 can tell you, and that is why your O2's jumping all over is normal). It takes an average of the low-high pulses and calculates a fuel trim until the number of high and low pulses are equal (=14.7 AFR). There is a STFT Short Term Fuel Trim that jumps around A LOT, and the 'average' of that ist the LTFT. Don't even bother looking at the STFT; there's no useful information from it. The PCM is constantly learning and adjusting LTFTs. The LTFT ranges are stored in the PCM's learning memory in fuel trim cells, which aren't of major concern unless you're tuning. Its simply a way for the PCM to learn FOUR situations differently: decel, cruising, accel, high accel.
Most untuned cars have LTFTs that are over +/- 5.
A simple weather change can result in LTFT changes. More humidity in the air will probably mean less oxygen per air mass/volume, so that PCM will (rather quickly) learn and adjust the fuel trim... it probably would go negative with less oxygen causing it to run a tad rich till the fuel trim caught up.
Tune your fuel trims?
Why tune your fuel trims when a fuel trim in itself is actually 'self-tuning' the PCM? A couple reasons... your throttle response will be better, and having a tuned fuel trim is the basis for the car running well overall, especially at high throttle.
To tune, you first need to do a lot of scanning (and logging), and then you end up making changes to your MAF tables (or AFC device). For powertuner users, there is excel spreadsheets to do mass-computations on your scans, to give you suggested changes to the MAF table... this is really the only way to get your fuel trims to "lock in" on zero, which is ideal. Non-tuner users can use an AFC to get the fuel trims close. This basically raises or lowers the MAF frequency... essentially shifting your MAF table. Advanced AFC's allow you to have different changes at different frequencies, so if your low-end is close to 0 LTFT and your high end is +10 LTFT (ugly), you can add some on the high end with a good AFC (its basically tricking the PCM into thinking theres more air coming in, so it adds extra fuel as well at first, then the LTFT learns its way down to somewhere close to zero).
It is very important to have fuel trims close to zero at WOT because once you go WOT, your fuel trim locks in.
O2 values
Two modes of thought here:
1) Non-WOT
We already talked about why the O2's jump around. There is absolutely no use looking at the O2 mV values when you're not at WOT or almost at WOT.
2) WOT
Your O2's should somewhat stabilize at WOT. Stock narrowband sensors are only accurate at 14.7 AFR... at WOT, your car is NOT at 14.7, its somewhere between 11 and 13.5 ish, depending on the power adder, tune, etc. This is why your O2 will stabilize in the high side, and usually can still provide us a little bit of information about our WOT tune; approximately how rich it is running:
N/A engines want to be in the 890-930mV range.
M90'd/GTP engines want to be in the 920-950mV range.
CSC'd engines want to be in the 900-930mV range.
These are wide ranges, and if you are tuning, you should refer to the powertuner forum for the specifics.
Where does KR come into play with fueling?
1) If your fuel trims are ugly, the delay in proper fueling may cause blips of KR here and there.
2) If your WOT O2 values aren't within spec, you could be running WAY to learn or rich, which will cause KR. Hopefully not too lean, thats dangerous.
example-
I once tried to tune a stock GTP with a fuel pump on its way out. It couldn't provide enough fuel pressure at WOT. I couldn't tell at first, because I don't have means of measuring fuel pressure, but it was obvious by the extremely low O2 values at WOT (and the car was very sluggish, and lots of KR). Had it been modded with a pulley, etc, he could've blown a piston out.
Killing KR with a tuner
1) Fix fueling in the problem spots.
I suppose non-tuners can somewhat do this with an AFC, but you really need a tuner, and I'll barely touch on how its done here:
PE (Power Enrichment) is a PCM mode for high throttle and WOT. There is a throttle % that activates PE mode (tuner adjustable). Then your PCM will stop using 14.7 AFR, and start using a new AFR specified in a PE AFR table (stock is somewhere in the 13's I think), mainly to prevent KR. Basically, your O2 values will tell you if you need to adjust the PE AFR tables. There is an initial table, and a "PE vs Time" table, which lowers the PE AFR by a specified value every second of being in PE. EXAMPLE: for a long WOT run like at the track, your initial WOT O2 values might be good, but if they keep going down, you need to adjust the "PE vs time" table so that it keeps lowering your AFR to prevent going too lean, KR getting out of hand, and blowing the engine. Rising cylinder temps can be blamed for this.
2) Timing
I didn't plan on talking about timing at all in this thread, but IF your fueling is all GOOD, and you're still getting KR, your timing needs to be reduced (this is assuming you M90 people aren't running to small of a pulley, have a clogged cat, undermodded, etc). With a tuner, you can find specific problem areas and reduce timing just in those areas.
Hope that helps some people understand and know what they're looking at a little more. Anybody please correct me as necessary, too.
Previous Topic
Index