re AF-XIED, as I understand it they can only richen the mixture (they cant lean anything out) - which on my hexhead would not be a sensible solution.
Actually, not quite so.
The AF_XIED (AF from now on) relates to one thing only, the voltage from the O2 sensor.
This means that any time the fuel is richer than selected setting (Richer means higher voltage emitted by the O2 sensor), the AF sends HIGH signal to the ECU (i.e the ECU thinks it gets a feedback from the O2 sensor that the fueling is too rich) , meaning that the ECU goes leaner. And the AF will keep sending the HIGH signal until the ECU has leaned enough to where the O2 sensor generates a voltage lower than the threshold.
This is the wonder of the AF, it base it's commands on a measured signal (from the O2 sensor), it doesn't just add a bit more and disregard the result.
Take a look at this curve, it's a scope readout from a 1200LC with a AF installed, blue curve reads the voltage from the O2 sensor and red reads the voltage generated by the AF and is transmitted to the ECU O2 signal input.
Look at every time the red curve peaks, and see what is then happening to the blue curve (voltage from the O2 sensor).
The fueling (and hence the AFR) keeps swinging between richer than and leaner then the reference voltage (or reference AFR if you like). As mentioned before, the ECU only cares about two levels from the O2 sensor, that is leaner or richer than the average (the value is 0,45 V, give or take a few mV, where 0,45V represent AFR 14,7, which is the target for emission control)).
Back to the graph: When the red curve is at it's lowest (the ECU reacts to the signal passing 0,45V either direction), the blue curve also changes direction. When red curve (signal to ECU) drops below 0,45, the ECU decides that the current fueling is too lean, and it starts adding a bit more, and you can see that the blue curve is again rising, meaning that the fueling gets richer.
After the fuel gets even richer, the blue curve keeps rising (voltage from O2 sensor), and it keeps rising until the voltage passes the defined 'high voltage' as defined by the AF, based on the setting selected by the user. Once the high blue line (high voltage) has been reached, the AF sends a too-rich feedback, i.e the (red curve)voltage rise to above 0,45V. And look at what happens when at the top peak of the red curve. By now the ECU has received a feedback that the fuel is too rich (as it reads the voltage from the AF (red curve), and once again, it starts to lean (blue line dropping).
The border for rich/lean (meaning blue curve top/bottom voltage) level is defined by the selected setting in the AF.
Here is an example where the AF is set to position 8. The reading was done a couple of days later, so the probes where switched.
So this time, the red curve reads the O2 signal and the blue line reads the AF output.
At this setting, the AF has defined a closer value between high/low level, and we can see the curve is more level.
A close to steady 0,9V (from O2 sensor, red curve in this chart) means an AFR 13,5 +/-,( ref. the chart published in my post a few posts back.)
So, to sum it up: The AF takes a positive control of the fueling, based on feedback from the O2 sensor. And it does so by maniplating the O2 signals to the ECU. Where the OEM mapping would make the engine run too lean, the fueling will become richer, and where the OEM fueling is too rich, it will be leaner. (The reference for too rich/lean is here defined by the user setting of the AF)