Does the ECM add or subtract fuel during PE mode due to the BLM value????

I know that for my '99 LS1 car, the PCM will add fuel during PE mode if the Long Term Fuel Trim (BLM to us 3rd genners) is positive, BUT the PCM will not take away fuel during PE mode if the LTFT is negative. FYI: The base line for FTLT is 0%.

 

In general what you have found with your '99 is what I have found with the 3rd gen ECMs. This is even true with the '747 truck ECM.

As 3rd gen f-bodies have used several different ECMs along with various maskid's I can not answer for all of them.

Even more interesting is how the 3rd gen ECM/maskid's handle the INTegrator or short term fuel trim. I know of one in particular that will even use the INT (locked at that value) in PE mode if it is found to be adding fuel.

RBob.

 

So, what we are doing with our '99s is to make the LTFTs (BLM for '87s) between 0% and -8% by calibrating the MAF table.

We do this (negative LTFTs) so that the PCM will not add or subtract fuel during PE. All PE fuel is gotten only from the PE tables, which is nice for tuning purposes.

 

blm in $32 $32b $6e i believe is locked ar 128 as well as the integrator 128 when entering wot or pe mode. think about it would you want you ecm to decide to subtract fuel at wot ?? i wouldnt and i dont think gm did eiether. however i think $8d locks blm at whatever it is then uses this as a flat correction for pe mode. i havent played with $8d enough or read enough about it to know for sure.

 

funstick,

My '99 PCM does not set the LTFT value (BLM) to 0% (128), when it goes into PE mode during WOT runs. The '99 PCM will add in extra fuel if the LTFT is positive (thats the save thing to do), if the LTFT is negative, such as, -10%, the PCM sets it to 0% and does not take fuel away at WOT because that may blow up your engine.

This whole situation equates to for the '87 ECM: if the BLM is above 128, the ECM will add extra fuel during WOT runs and if the BLM value is below 128, the ECM will set it at 128 and not take fuel away during WOT.

I have done several scans with my autoxray where during cruising around the BLM was 112 and during WOT the BLM was shown as 128. So if you have the ECM taking fuel away as an adjustment to the AFR during cruising around, the ECM will set the BLM to 128 (tempoarily) during PE mode so fuel is not taken away at WOT. I do not have any scans where the cruise BLM is above 128, while the WOT BLM is set to 128.