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BLM counts, what do they mean exactly?

20K views 5 replies 3 participants last post by  FBN Firebird Nation 
#1 ·
I just finished sorting through some ECM/sensor/PROM problems yesterday. Replaced the O2 sensor, removed the PROM adapter I was using and finally fixed the ALDL so I can use a scantool to see what's going on with the car. My main question is how do I interpret the BLM counts? I know that 128 is the goal of the ECM to maintain stoich or as close to it but when the counts go low or high numerically what am I seeing? Mine currently read 150-153, is that indicating the ECM is trying to lean it out due to the 22# injectors currently in use or what? The car runs great thanks to yesterday's fixes but this BLM issue is causing me concern.
 
#2 ·
The ECM calculates the fuel injector pulse width on the assumption that it knows all about your engine... displacement, injector size, mass air fow, etc. It uses an equation that includes the terms:

..... x (BLM/128) x (INT/128).......

The BLM is the long term fuel correction. This is what is stored in the ECM in volatile memory. The INT is the short term fuel correction, and its not "stored". The ECM recalculates the short term correction for every cycle of the calculation - about 9 times per second. It is intentionally forcing the closed loop A/F ratio to swing back and forth between very slightly rich (14.65:1) and very slightly lean (14.75:1). It has to do that for the cats to work.... when its lean the cat stores the excess oxygen, when its rich the stored oxygen combines with the unburned hydrocarbons (HC) and carbon monoxide (CO).

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.

For example, if your fuel pressure is too low, the injectors don't flow the amount of fuel the ECM is expecting, it runs lean, and the O2 feedback elevates the BLM's above 128. If you had a vacuum leak, same result. If you have an MAF sensor and its dirty and reporting less mass air flow than is actually entering the engine, its a lean condition. If you have a speed-density setup, and have modified the intake and exhaust to improve the engine's breathing, but have not had the programming modified with changes to the volumetric efficiency (VE) tables, it would run lean. These are all "TRUE" lean conditions.

But sometimes, the O2 sensors give the incorrect feedback, indicating a "FALSE" lean condition. The main examples of this would be 1) faulty O2 sensor(s), 2) misfires, 3) exhaust leaks before the O2 sensors. All of those trick the ECM into thinking the engine is running lean, and it adds a bunch of extra fuel, using the BLM's, that the engine does not need.

I have this online writeup that includes the A/F management aspects of the LT1 engine management system, but its pretty much the same for the 3rd Gen fuel injection systems:

http://members.aol.com/InjuneerZZ/ScanMast.htm
 
#3 ·
Thanks for the great info. I'll go back and look at it again after the drive Saturday evening which should have made a difference. The previous info was after reconnecting the battery for clearing the computer and replacing a possibly faulty heated O2 sensor. My INT was anywhere from 127 to 132(but mostly 128) during this time while the BLM was at 150-153. I'm looking into new injectors (strange how it's seeing a lean condition with 22# injectors on a 305) originally to have the "correct" 19# units to replace the 22#ers in there now. The car does have headers (shorties) and high flow runners along with a 3" catback and no cat at this time. I guess that would explain why it's seeing lean, would you agree?
 
#4 ·
As I recall, the 305 TBI engine used a speed-density system. When you swapped to the TPI, did you switch to an MAF system? What size injectors is the chip programmed for?
 
#5 ·
I stuck with speed density. The problems have been found and remedied. Got rid of the PROM adapter which got the car acting normal again. Then replaced the old 22# Multec injectors with a set of Bosch Design 3 injectors with the correct 19# rating and brought the fuel pressure back to 44 psi which has the car's BLMs back to a perfect 128. Thanks again for the info, I've saved it for future reference as I hope to get into burning my own chips at some point down the road when I do some more serious engine mods.
 
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