My O2 sensor voltage (in C/L) is cycling from rich to lean and back (I know, it's supposed to do that) BUT my BLM counts are NOT moving. However, if I give the throttle a quick blip, the BLM counts will briefly change then settle back to 123 or so.....without moving. Weird!

 

if BLM stays 128 then everything is good, right? I'm no expert on this stuff yet but from what I know you want BLM to stay as close to 128 as much as possible. Integrator moves with O2 crosscounts, not BLM. If BLM moves up or down then that means you are running consistantly rich or lean.

 

The ecm will make an estimate (based in prom data) on how much fuel the eng needs and it applies that mixture. When eng reaches closed loop, the computer adjusts itself based on what the o2 sensor is telling it. The blms reflect how much of a change the ecm had to make from it's initial estimate to what the o2 sensor says is right.

BLM numbers below 128 mean the ecm had to remove fuel from its' original estimate. Numbers above 128 indicate the ecm had to add fuel.

When you adj the mixture on the prom, you are changing the data the ecm uses for its' estimate. The closer you can get the estimated mixture to what the o2 sensor says is right, the closer the BLMs will be to 128.

There are many BLM cells. As the eng load and RPM change, the active BLM cell will move to another point on the BLM cell array. When you change cells, the learning starts over again. The BLMs for each cell default to 128. Once you have enough data points for that cell the BLMs will start changing to reflect what the ecm has learned. Thats why you will see BLMs of 128 as you transition across cells that haven't learned yet.

What you are seeing is normal. It's just telling you that based on the prom data, the ecms' estimate is a little rich. If you want to make the idle BLMs closer to 128, you'll need to alter the prom data.

An example: If the prom data tells the ecm the fuel injs are bigger than they really are, it will hold them open for less time, which will lean the mixture estimate and move the BLMs closer to 128.

Another example: By altering the MAF look-up table (it converts MAF signal voltage to grams/sec of airflow) you cam make the ecm think there's less air entering the engine and it will decrease the amount of fuel it delivers; making the mixture estimate leaner and the BLMs closer to 128.

Hope this all makes sense.

 

I found this VERY helpful - thank you for taking the time to explain!