Here is how I see it. My car has a BLM reading of 150-160 all the time. The sensors are telling the ECM that the car is running lean so it tries to offset this by adding more fuel. So if there is a vacuum leak after the MAF or exhaust leak before the o2 which the sensors cannot compensate for, the BLM will never even out at 128 no matter how much fuel is added correct? In this case, the car would be running overly rich giving poor performance, driveability, and gas mileage, correct? Thanks for helping out.

 

This was so good I just had to save a copy of it.

To quote Ed, he articulates so well, couldn't have done it better
myself, here is what he said in an earlier post. If you read this and
don't get it you never will.

When the ECM reaches the top or bottom end of the table it will
set a trouble code.

Your ECM does a calculation for how much fuel the engine needs. It takes sensor readings, and the cross references them to tables in the chip. In the case of your MAF car, it measures the airflow, then looks at the tables to see how much fuel it takes to get the right air / fuel ratio (AFR). It then injects that much fuel.

In closed loop, your ECM is also taking readings from the O2 sensor. Your ECM's goal is to keep the O2 switching back and forth around a rich / lean point, so that the average AFR is right on. Your ECM keeps track of the average amount of fuel it is injecting to keep the AFR around that rich/lean switching point. If the value for fuel in the table is making the O2 sensor show lean, your ECM adds more fuel. If the O2 is rich, it takes fuel out.

Now on to BLMs, the Block Learn Multiplier. Like i said, the ECM has tables in the PROM that tell it how much fuel to inject @ a certain RPMs and a certain airflow. These tables are divided up into 9 sections or BLOCKs. Also like i said, your ECM keeps track of how much fuel it needed to inject to keep the AFR correct, thats the LEARN part. The MULTIPLIER is how much fuel the ECM had to add or subtract in that BLOCK to keep the AFR right. Each BLOCK has it's own BLM, so that it will have one number each for low-mid-high rpms, @ low-mid-high load.

As you probably know, BLMs will be a number between 108 - 160, and 128 represents the 'ideal' BLM. Now let's say you have a scan tool and you notice all your BLMs are high, let's say they are all around 144. That means that your ECM is adding 144/128 more fuel to what the tables say you need. If the tabes say you need X fuel injected, BLMs of 144 means it is actually injecting 144/128 or 1.125 * X fuel. If your BLMs are 110, it is injecting 110/128 or 0.85 *X fuel. You can see why a BLM of 128 is perfect, 128/128 = 1, and 1 *X = X, wo that means the tables have the right value already.