I don't think you're fully understanding what vF is.
vF isn't your air fuel ratio, and it can't tell you what your a/f ratio is.
All the vF numbers can tell you is where you are on the ECU fuel correction map. The ECU uses the oxygen sensor to read the air fuel ratio during idle and normal driving conditions (light accel, steady cruise) when the engine is at operating temp. It then takes the oxygen sensor readings and tells the ECU what corrections to make immediately at that exact moment to get the desired 14.7:1 a/f ratio. That's what's called the short term fuel trim. It is a temporary way for the ECU to correct the fuel maps. The short term fuel trim is deleted every time you shut the car off, since it's designed only to make small, fast compensations.
If the ECU sees that you are always on the positive or negative side of the short term fuel trim, or sees that it needs to make drastic compensations, it takes the correction number from the short term "map" and moves it to the long term fuel trim "map". The long term fuel trim is saved when you shut the car off so that it will be close to correct next time it's started. The only way to clear the long term value is to disconnect the power from the ECU (example: disconnect the battery).
The long term fuel trim number is displayed through the vF voltage. Calling the fuel trims a "map" is a stretch, because there aren't multiple cells for the entire RPM range, there is a mass correction value (like "add 15% fuel") applied to all conditions. If there were multiple cells, we wouldn't need to worry about the fuel trims and vF so much.
Since it knows you need that change made during cruise/idle, it automatically assumes you need it during WOT and heavy acceleration, so it applies the fueling change to everything. This is where the problem lies. If your mods made the car run rich at idle, and not at 3000rpm cruise, then your fuel trims will float up and down depending on what your did last.
Imagine this:
You were driving around without stopping to idle and you used your wideband to tune your WOT a/f ratio to a nice and lean 12.5:1. Fine, perfect. Then you stopped at the longest traffic light of your life and let the car sit and idle. Your car idles 10% richer than at cruise. The ECU saw that, and leaned the fuel trim out while you sat there. You take off from the light and hit WOT... and your perfect WOT a/f ratio is now 10% leaner. PING PING PING... Not good.
So how do you use vF to tune and prevent that?
Like this:
So take the car for a drive on the interstate, cruise in 5th gear at the same RPM for a few minutes. Let vF settle where it wants. Then adjust your AFC LOW throttle map at all RPM cells so that it will put the vF at 2.5v (figure out the fuel change based on the vF diagram on this website
http://alflash.com.ua/vf1.htm). After the changes cruise in 5th at the same RPM as before and let the vF move and settle to 2.5v. Drop down to 4th to raise the rpm (maybe around 3500 to 4000), cruise for a few minutes and see if the vF changes. It shouldn't if your LOW throttle map is tuned right. If it does change, write down the RPM and how much the vF changed. You'll need to add that correction in your AFC at that RPM cell (in the LOW map!) later. Try to repeat this at every RPM block in your afc below 4000 RPM. Yes, it's a real pain in the ass. A dyno with an eddy brake makes it easier and safer, but that will cost you (about $300-$600 for dyno time). You will also need to do this with idle, because you want ALL of the RPM ranges that you use during normal driving to be adjusted so that vF stays right at 2.5V.
So again, the overall goal is to tune the low map on your AFC so that the vF number stays the same all the time, and never moves while you're driving. It should be 2.5V all the time. Note that it will drop to zero volts when: the engine is not warmed up, you are accelerating hard, or when you let off the throttle and the engine is decelerating.... that is all normal. It may also change up or down when you go to a very high or low elevation, or when the temperature outside changes drastically (like from zero degrees to 80 degrees... summer to winter). That's also fine. That's actually what the fuel trims were designed for, not to tune mods.
AFTER you tune vF and monitor it for changes for a few days of daily driving (or weekend cruising, whatever)... you can safely tune your WOT map (if you have an AFC this is the high throttle map) with a wideband. You can google how to do that, because there's a billion websites and posts about it everywhere. You need a wideband to tune WOT, because there is nothing built into the car to read your WOT air fuel ratio.
