Okay, here's how I believe all this crap works.

You're driving, let's say at part-throttle in closed loop. The MAF is reading a certain amount of air. So the PCM tells the injectors to squirt a certain amount of fuel that corresponds to that amount of air influx. Then, the motor combines air and fuel, burns it up, then spits it out the exhaust. The O2's read the amount of fuel in there. They say, "hey, that's a lot of fuel." So the Long Term Fuel Trims [LTFT] (BLM's, right?) start to go down, from 128 to something lower, let's say 110. Which basically means that you're taking fuel out.

Which means to me that the BLM's are "fighting" the MAF. Who wins? Or do they just continually fight each other??

Is all this right?

 

You have the understanding of the control logic, yes, except the O2 isn't measuring fuel, but rather leftover O2 from combustion. (lean means lots of leftover O2)

On a properly operating engine:

128 is ideal. The computer is allowed to range from around 100-160. 100 means that the MAF is reading way more air than is really coming in, then the injectors squirt too much fuel, then the O2's recognize this and pull fuel. The opposite is true if the BLM's are 160. If the MAF is reading properly, and the injectors are calibrated correctly, and the O2's are correct, and the Long Term BLM is 128, then you are at the ideal condition that is programmed into the PCM (14.7 a/f)

There are lots of other things that could cause the BLM's not to be 128 though:
Wrong injector size programmed in
Leaks in intake or exhaust(between the MAF and O2 sensors)
to name a couple...

Dan

 

Okay, I want to focus on one thing you've said...

"wrong injector size programmed in"

I just had my injectors cleaned and tested on a bench. He used 43.5 psi and says they are 26 pounds per hour.

If I'm not mistaken, the #/hr rate is a relationship to the psi you throw at it. Right?

So if I lower the psi with my AFPR, I'm essentially lowering the amount of fuel that is being squirted in the cylinder, right? Which in turn will lean me out, right? Which should show me LTFT's that are closer to 128 (since I've been running LOWER than 128 normally). Is all that correct?


Assuming that logic is correct, why wouldn't I just leave the psi at 43.5, then adjust something in the program? Such as the MAF tables...

I just keep thinking that at WOT, my injector DC levels are going to be getting to 80% or more... Which I just confirmed to be 87% in one of my datalogs...

So if I lower my psi, I get closer and closer to maxxing out my stocker injectors!

Okay, so my question is, should I adjust the PCM or the injectors via the AFPR?

 

If you're getting close to maxing out the injectors you can increase the pressure. You'd need to calculate the lb/hr at the set pressure and change the injector constant accordingly. I'd leave the maf tables alone.

 

The gm stock injectors (97SS LT1) 26#. They would be forced closed when the tank return side fuel regulator was blocked and fuel pressure spiked under max boost. This was typical of the stock injector design. I went with 30# saturated Accell injectors over the 26# stock. The pintel design is different and did not force closed with fuel rail pressure spike. That was a long time ago, and, I do not use that system anymore (went to... 383ci LT1 D1 72# Seimens Inj). I have adjusted the MAF cal curve for low speed closed loop mode. B.

 

Stock F-Body injectors, 94-97 are programmed at 24.9 #/HR.

If you change the injector size (e.g. increasing from 24.9 -> 26.0) you need to change the injector constant in the PCM. If you change the fuel pressure, it will change the injectors flow capacity. Since the clean/test shows your injectors flow 26.0 #/HR, that's the flow constant that should be programmed into the PCM. Alternatively, dropping your fuel pressure (no vacuum compensation) to 39.9psi would "resize" your injectors to 24.9 #/HR, making them the same as the stock programming constant.

Makes no sense to lower the pressure though. The correct step it to put the correct flow constant in the PCM. If you want a little more "headroom" (keep the duty cycle low), bump your fuel pressure a bit. Raising it to 50psi would allow them to flow 27.9 #/HR, for example.

 

Well it sounds like we're all in agreement here...

Now that I've got that cleared up - what would you guys suggest as the "first" step in getting the BLM's closer to 128?

 

http://web.camaross.com/forums/showthread.php?t=505190

Here is a sticky based on things I did years ago (look up top of this forum). Make sure and read the link inside this post!! I did this years ago and it has proven to work. You asked the question about getting to 128, well here is what did it for me in the complete RPM range, not just that range where you don't go into PE mode.

You seem to be delving into it enough to read all the details I have created inside. If you do, you will be much farther ahead than I was. I had to learn on my own.

 

Did you ever get the knock issue worked out? I posted that question in your thead, but never heard back.

BTW, I think your idea for PE tuning is a very good one. You just have to remember that your car is running 14.7 a/f while you are getting the information that you need, and don't forget to switch back

Dan

 

Dan sorry for not seeing it earlier.

I did a rebuild and my knock went away. the only thing I can think is that I didn't have a damper on the crank because the guy I bought it from took it off. Well, when I broke my second timing chain, I rebuilt it. It was then I found out how critical dampers are. I went out and bought the best one on the market!

Problem solved!