I have been trying to absorb as much information on tuning as I can and I was reading that when the BLM cell # is 16,17, or 18, the car is running in "open loop" or closed loop with learn disabled. I haven't logged the car driving it yet, but at an idle in the driveway, at "max" temperature (~170 *F), the BLM cell stays at 16. Does this mean that the tune for my car is set to run with the learn disabled all the time? The BLM is at 120 and the INT is at 128, which means that the a/f is rich at an idle, but the computer is compensating for it with the BLM. Correct? Do I get any gains from just adjusting the a/f to get the BLM to 128? Now, if I log the car driving and see the BLM is always w/in the 108-160 limit, the car should have the correct a/f, right? Now, at WOT, if the car is running rich, the BLM gets set to 128, so how do you know if you are rich at WOT? Any useful help with this would be great. I'm trying to learn as much as I can and it is extremely interesting to me. Not a whole lot of people can say that they can tune an efi car.

 

Cell 16 is the "idle" cell, whether its running open or closed loop. You can still see Cell 18 in closed loop.... sometimes the PCM wll use Cell 18 in PE mode. The PCM will also use Cell 17 for decelleration in both open and closed loop.

The difference when you are running in open loop is that you will NOT see Cells 01-15, the short terms (INT's) will never vary from 128, and if your PCM was reset, you will never again see a long term (BLM's) other than 128.

And, running open loop/ learning disabled, it means the the BLM's and INT's will never change, since they change because of "learning" (BLM's) and closed loop control (INT's).

BLM's between 108 and 160 means the PCM is able to keep the A/F ratio at 14.7:1 in closed loop. But it doesn't mean that it will run at the correct target A/F ratio in power enrichment (PE) mod, aka "WOT". If the BLM's in the high load Cells are below 128 (pulling fuel) the PCM will use a BLM of 128 at WOT. Under those condtions, PE mode would end up richer than the target A/F ratio set by the PCM. IF the high load cells are above 128, the PCM will use Cell 15 and the BLM's stored in Cell 15, meaning there is a better chance the WOT A/F ratio is where the PCM wants it.

You really need to identify the causes of BLM's that go outside the range 120-135, since that indicates a potential problem with the system components, or with the tune..

 

I haven't looked to see if the computer ever went into learn mode, I just assumed that cell 16 was idling for learning disabled. I'll check that Mon. when I get back home. So, when I log the car at WOT, lean it out until it is referencing cell 15 and not cell 18? As long as cruising/idling BLM's are w/in the limits (108-160) in the learning, closed loop condition, the a/f should be good? Are there any gains in getting these as close to 128 as possible? Once I check if it is learning or not and I log the car driving, I'll come back with more questions that I'm having a hard time finding answers for. I appreciate the help Injuneer.

 

It ALWAYS uses Cell 16 for idle, open or closed loop.
You lost me. You can't lean anything out except the PE mode target A/F ratio. The PE mode target A/F ratio does not affect the BLM's. The 14.7:1 used in closed loop/part load operation is hard coded into the PCM. You can't change it. There are ways around that, but its not normally done.

I think maybe you are asking if you should "lean it out" so that the BLMs will be above 128 and assure that you use Cell 15 in PE mode. I would think the other way is preferrable.... richen the setup enough to force the BLM's below 128. Then the PCM will always default to 128 for the PE mode calculations.

The closed loop A/F ratio will be correct, but you shouldn't see BLM's at the extreme limits. That indicates something is wrong.. either a sensor is not supplying good data, you have a problem like a vacuum leak, or the constants used in the tune are not correct.

Even if you have the BLM's at the limits, and it needs to lean it out more or richen it up more, it can use the INT's. But that's not a good way to run. You want the parameters and input signals used to reflect the correct conditions so the BLM's are close to 128.

As noted above.... keeping them slightly below 128 assures a more consistant PE mode A/F ratio.

All of this is "theory"... I haven't used the stock PCM in my car since 1999. I am not knowledgable in tuning the stocker. I do my tuning on a MoTec, and run speed-density open loop.