If the ECM finds that the long term average of the INT's is adding fuel (over 128), it ratchets up the BLM's to try and get the average of the INT's back to 128. It stores those values in the volatile memory..... that's most of what the ECM "learns". If the BLM's are above 128, the ECM is responding to what it BELIEVES is a lean condition. IF the BLM's are below 128, its responding to what it BELIEVES is a rich condition.

The BLM's are developed during part throttle/part load conditions, when the ECM is operating in closed loop (O2's are being used to modify the fuel rate). They are not learned when you are running at WOT, so they are not an indication that the engine is leaning out at peak power output. They are used at WOT to calculate the fuel rate (but only if they are above 128). A BLM above 128 indicates that something in the fuel system is not responding the way the ECM thought it would.

If the air/fuel mixture is too rich for the engine while you are running at cruise speeds, the engine may tend to load up and foul the spark plugs, while if the air/fuel mixture is too lean the engine may misfire at idle and at light loads or tend to run hot or overheat.

Lean Symptoms:

Poor power output
Hard to start when cold – requires excessive choking.
Lengthy warm-up required.
Runs better (but not good as it warms up)
Spark plugs overly clean..with no deposits or slight glazed appearance
Rough, erratic idle that drifts toward higher rpms than expected based on the setting of the curb idle screw. Sometimes a lean condition will cause a “hanging” idle that is slow to settle down to set idle speed.
Backfiring
Sluggish…hesitates when the throttle is opened, then recovers (often accompanied by a slight backfire)
Vague throttle response
Surging at steady throttle cruise operation
When fully warmed up, runs better when choke is applied.
Engine runs hotter than normal. Headers can turn cherry red in extreme cases!
Temporarily removing air filter element makes the engine run worse.
Unusual “sucking” noises in the intake area.
Runs better as you climb to higher elevations
Slight back-firing on deceleration. This normal tendency is controlled by the air cut-off valve(s) operation. If the air cutoff valve(s) is working properly and you still have backfiring on deceleration, something is causing a lean condition.