How does one tune the VE tables when BLM's are split? Normally if BLM's are both say 140 (lean) you can take 140/128=1.093 then correct the appropriate VE table using KPa and RPM to find the number to change. But if one BLM is 140 and the other 118 then what do you do? Which number do you use? How do you tell if you need to compensate by adding fuel or taking some away? I have some cells with almost perfect BLM's, very close to 128 but others are way off and I have no idea how to tell if the car is rich or lean at those places. I have a wide band in the pass exhaust header and it staes pretty close to 14.7 so I am lost as to what to do to find out where I need to go with changes.

93 Vett, hot cam, Trick Flow 185 heads.

 

I too have a 93 w/ a cam in it. I used to have the hotcam, and same issue.

The issue did not go away wih the 224/236 114 LSA cam I recently put in it.

Since we dont have bank multipliers. The best you can do is 'split' the difference to make it such that one bank isnt so bad.

Your example.
Left bank 140 (140-128 = 12)
Right bank 118 (128-118 = 10)

I guess you could make it so that left bank = 139, and right bank = 119, such that you 'only' have 11 points differece from 128 on your left bank...for a whopping improvement of 1 point closer to 128 ....Kind of a bad sitution.

IMHO the o2 sensors are picking up some of the reversion (or unburnt air/fuel charge..though they only care about the air part of it) that goes out your tailpipe with that cam. They are trying to base their fueling off of that. Thats where it gets messed up.

Its not from 'false air', 'vacuum leaks', or any of the junk people have been telling me on here for years.

I suspect this condition has robbed me of fuel economy and has contributed to a nasty idle smell.

The newer OBDII, Mass air flow systems 'seem' to do better with these types of tuner cams.... Though for big HP, it seems everyone comes back to speed density. ...

I did hear about someone setting their O2 rich/lean threshold to 450 mV (vs. 500 mv) in tunerpro using a $DA2 definition file. They claim this works for cars that are just dumping excess fuel/air (like mine did) at idle.

I have yet to try this...

 

Do you also have a narrow band O2 sensor on that side? I have heard that the ECM isn't compatible with wide band O2 sensors. I have seen reference to a "kit" that makes a wide band look like a narrow band to the ECM.

Do you have heated O2 sensors?

Referring to changing the rich/lean threshold: I don't think that would help, as it might be OK when going in one direction but it would be "farther" to get to the threshold when going in the other direction. From looking at the narrow band O2 sensor curve, it pretty much "snaps" from rich to lean and vice versa (the vertical part of the curve):

 

Thanks a bunch for the reply's. I can run data master and log my car, as I watch the BLM's where they are close to 128 they stay that way and the car runs "good". But when I get to a cell where the BLM's are off if I keep the rpm's and KPa the same then the BLM's just keep getting worse until they are way off and I can even feel the motor start to miss. I believe what is happening is that the fueling is off to start with in that cell and the extra fuel from the side that is rich (batch fire on 93) drifts over to the other side so the computer pulls fuel on that side, it just compounds and gets worse until it maxes out the INT's. To me when BLM's get that far off there is no way to really tell where the fueling is or which way to go, that's what the computer is trying to deal with and it can't figure out what to do either.
I have read for hours on this problem and found no one with a real simple good way to fix this problem except for a guy running in open loop all the time. Or people who spent hour after hour logging/tuning until they happen to get it right.
I also notice that at idle my split's are the worst and at WOT (open loop) they are gone and the car runs great, I love the power of the hot cam and the TF heads when it's WOT but idle and cruse is another thing - annoying! More later.

 

Diz, couple of questions.
You say; "I too have a 93 w/ a cam in it. I used to have the hotcam, and same issue. The issue did not go away with the 224/236 114 LSA cam I recently put in it." Did you ever feel like you were making any progress with the hot cam and fueling? And is it better now with the 114 LSA of the new cam as to the 112 with the hot cam? I feel that the 112 LSA of the hot cam is better for Tq. but worse for idle quality. It seems like a cam with 114-116 would idle better. Also is your motor a 350 or 383?

And a comment. You also said; "I did hear about someone setting their O2 rich/lean threshold to 450 mV (vs. 500 mv) in tunerpro using a $DA2 definition file. They claim this works for cars that are just dumping excess fuel/air (like mine did) at idle." I suppose the idea here is to raise the 02 median mV so that it adjusts for a little less fuel? Am I right here?
I am thinking of splitting the pass side 02 and running one 02 with one wire for the left bank and the other for the right bank to the computer then tune with my wide band only since I can't make any sense out of the wide split BLM's. That would at least eliminate that problem and allow me to tune until I can get things right and then go back to two 02's, any thoughts?

 

Cliff; I have both narrow band 02's mounted in the header collectors, they are heated. I also have a wide band (Inovate) in the pass side collector about 10" behind the narrow band 02.
Looking at your chart I think you are right about changing the mV threshold to 450 would not make much difference, the "curve" at that voltage is vertical, so that looks like it would not matter much.

 

The O2 sensor output pretty much snaps back and forth between roughly 200mV and 800mV. You can see that in the datalogs. The only way I think adjusting the threshold value would help is if you were trying to use a wideband O2 sensor and faking out the ECM into thinking it was a narrow band sensor. A side note: I don't remember ever seeing what a wideband O2 sensor curve looks like...

Related to the above, you want to see O2 sensor cross counts constantly increasing. My clunky old ECM increases cross counts about every 1 to 2 seconds. If the sensor is stuck at rich or lean then you have a problem. That's going to affect the INT, which will then push your BLM off into la-la land. My INT bounces all over the place, so I don't consider that parameter worth watching. It's the BLMs that are important. Don't forget that the BLMs will be a little higher (like around 135 or so) if you have some ethanol in your fuel.

I suggest making a BLM chart like I did. I know they changed the RPM thresholds in later years so you'll have to make your own chart:

That will tell you which RPMs correspond to the BLM cells where you're seeing a problem. Then you can tweak your fueling tables for the appropriate RPM for those cells.

 

Cliff,
"you want to see 02 sensor cross counts constantly increasing" do you mean constantly getting higher and higher, like going from 200 to 300 then from 300 to 400? Or do you mean the 02's cycling from low to high and then back again? If the later I can see them constantly
cycling back and forth, I think they are both good. I have several cells where my BLM's are almost perfect like cells; 12,5,18,0 but most of the others are way off. Some are rich on the drivers bank/lean pass. while others are reversed. That makes me think it's not an 02 problem. I also switched my injectors from one side to the other side and it made no difference.
I know there is ethanol in all the fuel here, never thought of that pushing my BLM's a little higher normally. Does that mean then that readings at 128 are actually rich?
You talk about the INT's being all over the place but the BLM's move a lot also, that's why I made the post above, I am asking which BLM reading do you use when they drift all over the place, getting further apart all the while?

 

The O2 sensor cross count is incremented every time the O2 sensor switches from rich to lean and vice versa. It's only an 8-bit value, so it counts up to 255 and then rolls over to zero and starts over. If it's constantly increasing then that means that the ECM is controlling the AFR. The ECM richens the mixture and then waits for the O2 sensor to report a rich mixture. Once it does the ECM leans out the mixture and then waits for the O2 sensor to report a lean mixture. This process (switching back and forth between rich and lean) goes on continuously while the engine is running. It has to do it this way because there is no way the ECM can hold the AFR in the center of the O2 sensor curve.

When you do an ECM reset it sets all the BLM cells to a default value (on my car it's 120). If you look at the BLM chart I posted earlier you will see that it is virtually impossible to get into the "corner" cells (3 & 12). Take cell 12 for example: you can't have an airflow of over 34 gm/sec at less than 700 RPM. Basically airflow increases with RPM, so you'll find the engine spends most its time in the cells on the diagonal: 0, 5, 10, 15.

About the INT: This gets kind of complex... Background info for you to help you understand what's going on. [This all applies to my antique 1227165 ECM -- the newer ones are faster and other things are improved.] The ECM has 16 "functions" that it performs. It takes 1/10 second to do all 16. One is calculating injector pulse width, another is calculating spark advance, etc. Some it does twice in each cycle. I don't remember if calculating INT is done once or twice per cycle. It keeps going through the cycles until it finishes 160 of them (which takes 1 second -- thus the 160 baud data stream rate -- the newer ECMs switch to 8192 baud on command from a scanner) and then starts over. Anyhow, the ECM keeps a running average of the O2 sensor voltage. It takes an O2 sensor reading, throws away the oldest of three readings and adds the new reading and then averages the three values to get a new value. It calculates the INT from that value. Now the ECM compares the INT to the value in the current BLM cell. Things start to get a little complex here... ;-) The ECM does what is know as a "lag filter" on the BLM cell value. What that means is that it adjusts it based on how far away from the BLM cell value the INT is. If it's pretty close it doesn't change. If it's a little farther away it changes the BLM a little. If the INT is really far away it changes it a lot. The change amount is proportional to the difference between the INT and the BLM.

All of the above is necessary because of what comes out of the O2 sensor. Here's a pic of the O2 sensor voltage from my car at idle. Each vertical line is 1/2 second apart and each horizontal line is 200 mV apart. Zero volts is where the little arrow on the left is. The signal varies between about 200 mV and 800 mV.



This is why the INT jumps all over the place -- it's calculated from the O2 sensor voltage. The above is at idle. You can imagine what happens while you're driving, shifting, etc. That's why the ECM just gives up at WOT and goes open loop -- it can't keep up because the changes in the engine sensors are happening too fast and are too great.

Back to your situation: As I said earlier, if the cross counts are increasing then the ECM is controlling the AFR. That's the same as saying that the O2 sensor voltage is switching back and forth continuously, so it looks like you're OK there.

About BLMs: On my car the BLMs are constrained to a low limit of 108 and a high limit of 160. If the BLM hits the limit then the ECM just stops adjusting it and holds it at the limit. This prevents the AFR from getting too far out for the engine to run. It's a failsafe mechanism to keep the engine running even if the ECM is getting faulty sensor data.

About "128": This is the "ideal" BLM for gasoline at 14.7 AFR. If you're running 10% ethanol (which the engineers who designed this system never heard of) then the ideal AFR is 14.5 and the ideal BLM will be a little different. 128 is just a number. It doesn't mean anything special by itself. The main thing is that you don't want to hit the limits. It's nice to get the BLMs near 128 because it gives the ECM more room to adjust the AFR, but not absolutely necessary.

About split BLMs: This means that one bank not running at the same AFR as the other bank. Why is a good question. How to fix it is another good question. This is beyond my area of expertise, so I can't help you with this. I saw a LONG thread about 10 years ago on this very subject. I believe it was on the Buick Grand National (turbocharged V6) forums. Bruce "Grumpy" Plecan (RIP) was the expert over there.

This post has gotten ridiculously long so I'll stop now.

 

Cliff, lots of good basic info above, really appreciate you taking the time to write it all!
Here is what I have found out. I split the narrow band 02 wire to the computer (pass side, where wide band is too) and ran it to the driver side 02 out put wire. Now the computer sees the same 02 reading for both banks and split blm's are gone. I can now see that I am rich at idle and all cells below 2000rpm. I had thought that was the case but with blm's always reading split and something like 140/118 I thought it was lean so I kept adding fuel. Now I clearly see I need to take some out. When I can log and get all my blm's close to 128 then I will hook the other 02 back up. I have never heard of anyone doing this but it seems to be working and allowing me to get a true reading on A/F ratio that even my wide band did not show. We will see if it really works out.

 

Any update on your results?

 

Thanks for asking. As for the VE tables, I have been able to make great progress getting almost all cells close to 128. I am having a problem with timing (I think) now. My advance at idle is 28 or higher. The bucking has come back and the car is no fun to drive below 2000rpm. I am trying to reduce timing in the "main spark timing vs RPM vs Load" table but seem to be unable to do it. I have another thread about the timing issue, it's really hard to find any help with tuning. If I could reduce the timing I could at least tell if that is the reason for the bucking, I really don't think it's fuel related. I would like to rule out timing and then be able to look somewhere else for the problem.

 

Hey Tom, I posted in your Timing thread