swerve-driver

Can you let me know if my interpretation of BLM operation is correct? I searched and read quite a bit, but want to make sure I am clear on this.

A BLM of 128 means current air:fuel ratio values are acceptable and the ecm is relying on the fuel map(s) VE1 and VE2 control fuel into the engine.

A BLM less than 128 indicates that, based on current inputs, the motor is running rich and the computer is working to reduce the fuel going into the engine.

A BLM greater than 128 indicates that, based on current inputs, the motor is running lean and the computer is working to reduce the fuel going into the engine.

If wrong, any clarification is appreciated.

I have a few questions based on the above statements, but wanted to make sure I am looking at this the right way first.

As always, your input is appreciated-
S-D

Dewey316

that is the way i understand it, but i am not a pro at all, i am most likely about the same place you are, i have been dataloging, and am getting ready to start burning my own chip!

V8Astro Captain

iTrader: (1)

You are correct.

Carl

I will admit my ignorance up front but how can this be "correct":

"A BLM less than 128 indicates that, based on current inputs, the motor is running rich and the computer is working to reduce the fuel going into the engine.

A BLM greater than 128 indicates that, based on current inputs, the motor is running lean and the computer is working to reduce the fuel going into the engine. "

This states that if the BLM is greater than OR lesser than 128 the computer will work to reduce fuel.

I would think that if it is running rich the computer is working to reduce the fuel.

Not trying to be smart or flame or cause problems and could be totally wrong.

Dan W

If in the second statement you change the word reduce to increase you would be correct.

swerve-driver

Thanks for the input, guys.
Carl, you're absolutely right, good catch. I had copied and pasted and forgot to change "reduce" to "increase".

I have a further question concerning BLMs and their operation:

How is the required change in engine fuel requirements (represented by the BLM value other than 128) acted upon?
Does the ecm adjust fuel by multiplying the current VE value by some calculated amount? Where/how is this stored (if it is)?

I suppose that, if running on the highway at a set MAP and RPM for an extended period of time, the ecm could adjust fuel so that BLM would be 128 (or reasonably close). Then when MAP and RPM change, the ecm goes to work on the new RMP-MAP condition if needed. What happens to the fuel settings that were established at the previous RPM and MAP?

This is part of the process I don't understand.

If you guys can shed some light, I'd be thankful.

Dan W

There is table the ECM uses I think it is 3 cells by 3 cells. The cells do not exactly coincide with the fueling table that has many more cells... in other words there is not one BLM stored for each fueling table cell. Only 9 BLM corrections are stored (or maybe the table was 4x4 in that case 16) no matter for the sake of this converstion, the point is as you trasition from one fuel cell to another adjacent to it you may be working from the previous BLM or one from an unrelated cell. This is one of the reasons to get your fuel cells as close as possible since it takes the BLM a little while to change to the correct value when you start at the wrong one.

the 129-255 range for the blm equals a 100% change in fuel or each point is about a .8% change... so a 141 BLM would be commanding a 10% increase in fuel in the fuel cell being referenced at that time.

I hope that answers your question... I saw a post about this on the prom board maybe a year ago... I think the 7747 code was being discussed at the time.

swerve-driver

OK Dan, that helps point me in the right direction. I'll try to find that post.
Your help is appreciated.
S-D

V8Astro Captain

iTrader: (1)

In the ECM there is a BLM and an INT...correct? The INT is the "right now" fueling change. The if the INT is below 128 for a certain amount of time, it will "pull" the BLM down with it.

Lets say the ECM is seeing a rich condition and the INT goes to 118. This 118 INT will pull the BLM down to (example here) 120. Once the ECM sees that the BLM of 120 corrected the fueling the INT will eventually stop falling, indicating that the BLM of 120 is correct for that cell.

The actual amount of fueling change is dependant on alot of things. Alot of it is based on O2 error and airflow.

Dan W

Yeah, the integrator is the short term trim that drives the BLM long term fuel trim. Intergrator is based directly on oxygen sensor output and BLM is based on integrator.