Quick question. Ideally, what kind of fuel trim values do you want to see at WOT? Close to the 128 range or higher indicating that fuel is being increased when demanded? Just curious. Thanks.

 

Ideally 128...but it's better to have it lower. That would mean it's adding fuel. Higher than 128 and it's pulling fuel.

 

Some bad info here..... the BLM is a "multiplier", in effect the number that is divided by 128, and used in an equation to calculate pulse width. If the BLM is 128, the resulting multiplier is 128 / 128 = 1.00 and the fuel delivered is exactly what the PCM is calculating based on the data it has been programmed with (e.g. injector size) and the data provided by the sensors (e.g. mass air flow). When the O2 sensors detect a "lean" condition, the PCM increases the BLM's to ADD fuel. A BLM of 135 for example would result in a multiplier of 135 / 128 = 1.0547, and the PCM would be increasing the fuel delivered per injector pulse by 5.47%.

A BLM higher than 128 adds fuel. A BLM below 128 reduces fuel.

The ideal BLM is 128. That means all the data programmed into the PCM is totally correct, it is making the correct fuel calculations with no corrections required, and that there is no unmetered air flow, and the injectors are performing as they are supposed to. You do not want a BLM more than say 5% above (134) or 5% below (122) the "normal" value of 128. That would indicate there is something that is causing the PCM to seriously miscalculate the fuel flow, or that the O2 sensor are not reporting the correct data, or that the system has air flow that is not being detected, or that the injectors are not delivering what they are supposed to.

A BLM higher than 128 would NOT be "indicating that fuel is being increased when demanded". It would be indicating that under that particular combination of RPM and load (MAP) the computer was not providing the correct amount of fuel. When a richer mixture is required, the PCM calculates a new "target" A/F ratio, again based on things like RPM and coolant temperature. If everything is working right, the BLM's for all cells will be very close to 128.

 

Fred's right. Not sure what I was thinking last night.
A BLM higher than 128 adds fuel. A BLM below 128 pulls fuel.
Just had it backwards.

But Fred....at WOT there is no fuel corrections being made, correct? So you'd want the blm's to be lower than 128 rather than higher....that way the car is richer than it would be with the blm's locked at 128, right? If the blm's are above 128 and the computer isn't making corrections than the injector pulsewidth is lower than it needs to be and the car would run lean. That make sense or have I misunderstood this?

 

So lets say that I do have values higher then 128 or even 135 (i've actually seen them up to the 150 range), could it be my ported MAF causing this? "Unmetered air flow"? Just got Tunercat and will be starting to tweek. My PCM has already been programmed by someone else, so i'll be building on that. My question is where exactly would I start making changes (what parameters) to get closer to 128? Also, I currently have my fuel pressure cranked up to 50 PSI. I had to do it on the dyno before getting Tunercat, just to get my A/F ratio down to about 12.5:1 from about 16 to 17:1 after I put a set of heads on. Would I be able to tweek the program to get my A/F to stay at around 12.5:1 and drop down my fuel pressure, or should I just leave the fuel pressure up there? I've got a set of 24# SVO's in there right now also. These are also values at WOT by the way incase anyone was wondering. Thanks in advance. Great info for a rookie like me.

Ken 95Z

 

I would get the correct constants and offsets for the SVO's then drop the FP to stock, I would also get a stock MAF on it, or scale your MAF tables. I have found it works better to do ONE change at a time to the tune, keep notes, lots of notes, and datalogs.
I put a set of billet MAF ends on my car and the BLM's shot to 140+. I may play with the MAF tables, but I am prepared to return the MAF to stock.

 

Depends.

If the BLM's were higher than 128, indicating the PCM was adding fuel, then the BLM's from Cell 15 (highest rpm/MAP combo) would be used in PE (power enrichment mode) to correct the fuel flow calculations. (Just to clarify.... when you go WOT/max load, you enter PE mode, and the feedback function of the O2 sensors is eliminated. But the fuel corrections MAY still be used.)

If the BLM's were below 128, indicating the PCM was reducing fuel, then the BLM's from Cell 18 would be used, or they would be set at 128. Hence, the PCM would not be using the BLM's in the fuel calculation.

I would assume this is a conservative engine managment strategy, to prevent damage. If the engine would be running lean without the corrections, use the corrections to insure you have enough fuel ar WOT. If the engine would be running rich without the corrections, don't use the corrections to make sure it doesn't run lean at WOT. That is just an assumption on my part. I honestly don't know why GM picked this strategy.

In any case - FOR STOCK PRGRAMMING - the last thing you want is to have the engine run any richer than it is programmed for. It is already programmed way too rich from the factory at WOT.

This is why you want the BLM's to be 128, confirming the PCM has all the correct data programmed in it..... injector constants, offsets, MAF calibration table, etc.

 

Isn't this the exact reason that it is easier to lock the blm's in cell 15 to 128? That way the computer isn't taking the blm's into account to get injector pulsewidth and you'd get a consistent a/f ratio @ WOT.

 

Fred,
I have a problem with my BLM's being too low. For some reason, after I did my M6-A4 swap and my new program, the L term BLM's are hanging down at the min. setting of 108, the S term BLM's are bouncing around but are usually below the 128 mark, I made no changes to the engine when I did the tranny swap, I changed the engine wiring harness from the M-6 to the A4 harness. When I wrote the new program for the swap, I used the same engine values from my old program to write the new. Before the swap on the old program, I had it running around the 128 # most of the time. Now it's running very rich. My O2's are about 980-1000 at wide open throttle.
I have tried taking a stock 94 A4 program and running it, and it was alittle better, but not much.
Anyway, I was wondering if you had any advise on how I could straighten this out. I have my program and current data logs if you would have the time to look at them.

Thanks in advance.

Brian

 

DanK
Just because you are consistent doesnt make you correct... Say you are running lean (for whatever reason) Your ecm compensates by raising your blm's. If you lock your blm's at 128 at wide open throttle you will be running lean. This could add up to big problems. Bottom line is you need to find out why your blm's are high and fix that problem, then you will see your blm's stay at 128 during WOT.

 

Let's try this again. Let's say you lock your BLM's at 128 in Cell 15. So, there is no "correction" at all.... the PCM is setting the PE target A/F ratio, and making all the calculations to get fuel flow, based on air flow. If the PCM has all the correct input from the sensors, all the correct program settings, like injector constants and offsets, everything is fine. The PCM is achieving its target A/F ratio at WOT, and since it is achieving the 14.7:1 target in closed loop, all your BLM's in the non-locked cells are 128.

Then something happens to screw things up..... the PCM determines, via the O2 sensors that the engine is running too rich using the programmed values and the sensor inputs, and for the non-locked cells, it develops BLMs less than 128. This puts the closed loop A/F ratio back to 14.7:1 where it belongs. Then you go WOT with a locked Cell 15, and your car is running rich, because the inputs to the PCM are not correct, or because the programmed data is not correct.

So... if you purposely pull the BLM's down below 128, then lock the Cell 15 values, you will be running richer than the PCM is targetting at WOT. I can't see any benefit in this.

If you just put in the correct data, and insure all your sensors are accurate, you will get close to 128 BLM's, and if there is an upset, caused by some factor you aren't aware of, the PCM will make the required corrections in both closed loop and at WOT.

Maybe I'm missing some feature of tuning with the stock PCM, and I'll plead ignorance.... I run a MoTeC M48Pro.... but it seems to me that accuracy is more important than anything else in insuring consistant results.

 

If you can send me the logs in Excel, or in .csv format, I will try and find some time to look at them. It seems odd that a stock A4 program doesn't cure the problem, so that would lead me to something getting damaged on the tranny swap.... O2 wires messed up, a harness wire pinched, etc.

 

Injuneer, your e-mail wouldn't work, if you drop me a line @ tko5o@hawaii.rr.com, i'll send you a log.

Thanks
Brian

 

Injuneer and sweetassz:
I understand everything both of you said.
Maybe I'm trying to make this more difficult than it really is.
But Fred, you answered everything I asked and I thank you.