Please help me understand what these LTFT's mean

[daniel76309]

I am new to this, reading everything I can, and trying to make sense of it, would appreciate any help/advice.

My car is a 1970 Camaro, LS 376/480 GM crate engine (cam specs are 219/228 duration, .525" lift), using GMPP controller package (E67 ECM, and as I understand it, an OS unique to the GMPP crate engines). The attached log was made with the original unaltered calibration from GMPP.

I am using an NGK AFX wideband which has been calibrated in free air according to instructions. The engine has long-tube headers, and all the O2's are in the "collector" area. The air intake is 4", fed by a small scoop below the radiator, and there is an airflow straightening device upstream of the MAF sensor.

I have attached a screen shot of a representive area I would like help with. The conditions are light throttle cruise at around 60 mph. I see AFR's from the wideband in the low 14's--I would guess that an average value would be about 14.3. At the same time, LTFT's are +9-10. If I understand this correctly, this means that the factory O2 sensors are detecting a lean condition and "asking for more fuel". However, if my wideband is correct, with the LTFT's adding fuel it is actually running overly rich. I intend to "calibrate" my MAF sensor using the technique outlined in Dan Maslic's book, but before I do that, I am thinking I need to know that I can trust my wideband O2 sensor.

Any comments or suggestions?


OE WOT + hwy cruise.hpl
Original tune 6-15-13.hpt
6-15-13.cfg

[Iam Broke]

~14.2 is an average AFR in CL for E10 fuel. I suspect your WB is OK. Most fuels are oxygenated in the US now.

Since you are learning to tune, learn Lambda and forget AFR, you will be happier in the long run.

In CL regardless of fuel type the Lambda sensors adjust the mixture to the stoich of the fuel, 1.0 Lambda. ~14.2 AFR for E10 gasoline.

If your WB has a display mode for Lambda, see if it's 1.0 at cruise. Most bounce .97 - 1.03 in CL.

[daniel76309]

Thank you for the quick reply. I am in fact burning fuel that "contains up to 10% ethanol".

I don't think my wideband has the option of displaying lambda v. AFR, but I will double check that.

I also need to go back and study up on Lambda... I seem to remember reading a long thread that dealt with that subject, and my "takeaway lesson" from that was that the wideband was not really reading "AFR" per se, and that it would say "14.7" at lambda, REGARDLESS of the fuel composition. Maybe I misunderstood. If I can find that thread I will post a link.

Also, much to my surprise, I recently found a place not far from here where they have ethanol free fuel. With that I could prove it to myself without question, one way or the other.

Thanks again for the info!

[Iam Broke]

Some widebands let you input the stoich value of the fuel you are burning so that you can display a correct AFR if you want to. You are right in saying a WB with an AFR at stoich of 14.7 will display 14.7 regardless of fuel when running at stoich. WB's really measure Lambda and display what you tell them to in AFR.

Lambda is much simpler when running blends and E85. My AFR at stoich in the table for E85 is around 9.8 but Lambda still = 1. My PE lambda is still .78 or 1.28 in GM PE terms.

[daniel76309]

Thank you for the info. I found the thread to which I was referring. I just reread it, and now I have a headache.

http://www.hptuners.com/forum/showth...-and-E85/page4

So if I am understanding correctly, when my wideband is reading 14.3 it IS in fact running a little rich of stoichiometric, even though it is E~10.

If so, that brings me back to my original question of why is this happening? i.e. why are the fuel trims kicking in and adding more fuel when it isn't being "commanded"?

[Iam Broke]

Perhaps the switchpoints of the O2's are a bit off or the sensors are getting old/contaminated. Hard to say. Some ECU's let you change the switchpoints of the O2's to compensate, but when I tried on my old WS6 to balance the banks with it, it didn't help. Perhaps someone else can help. Moving the sensors farther away with headers can change things too as the delay gets longer.

[daniel76309]

OK, thanks.

I wouldn't think that either the factory O2's or the wideband would be getting old or contaminated--I only have about 6000 miles on the engine, and much less on the wideband.

I read a recent thread on the effect of longtubes, i.e. the effect of delay from the fuel trim at the injector to being read by the factory O2's. I got a headache from that too, but I guess I need to take another look at it.

Would you think it would be relatively safe to assume that the wideband is giving an accurate AFR reading, such that I could/should make whatever ECM calibration changes are needed to make the "commanded" AFR equal to the "actual" (i.e. as reported by the wideband O2) AFR?

Is there any way (short of buying another wideband) of testing the one I have, to verify that it is in fact accurate?

[decipha]

yes you calibrate a wideband using narrowbands but in order to do so you must have accurate narrowbands = new narrowbands, tuning with old narrowbands is foolish

if both banks read the same approximate LTFT then you can ASSUME the narrowbands are reading accurate, however, its always best to put new narrowbands in a vehicle before tuning so you have an accurate reading and you know exactly where stoich is

disregard the fuel grade your using since that is irrelevant even if you were using e100 it wouldn't make a difference, your wideband should be spitting out 14.5-15.2s during closed loop cruising conditions, if its not then you need to tweak the wideband transfer so that it does

as it sits now your wideband is reporting 2% rich, (14.63/14.3), nothing to really worry about if you ask me

[daniel76309]

as it sits now your wideband is reporting 2% rich, (14.63/14.3), nothing to really worry about if you ask me

Thank you, you make a good point that it isn't all that much off, and that last statement got me to thinking that it is probably "safest" to assume that my wideband is slightly in error, reporting ~2% rich. Another way of looking at it (I think), would be that when I am at Lambda, the wideband will read ~14.3. The way I am understanding it, if I am wrong is assuming this, the worst case would be that I the calibration I end up with will be a tad on the rich side, which would be safer than going too lean.

[EagleMark]

2% is not a lot but I think I found your LTFT issue. I'm only on HP demo and not familar with this OS and use another couple programs so correct me if I'm wrong here.

E 10 fuel in most states says up to 10%, so 10% is 14.13 AFR or 1.0 Lambda. If only 8% then 14.3? and 1.0 Lambda. So 14.3 could be correct and the only way to know for sure is with an ethonol test kit. Still not the issue.

Your stoich AFR table in tune starts at 14.68 and changes all the way to 8.9?

If the Stoich AFR or desired is 14.68 and fuel is 14.3 or anything but 14.68 then fuel trims will be off to match because narrow band O2 is going to adjust to Stoich of fuel in vehicle. So start with setting the Desired or Stoich AFR in the tune first.

I would trust WB readings before the NB... now you can screw this up to on some controllers and or get it dialed in correct by changing the WB controller. If you have one with no adjustments then 14.7 AFR is 1.0 lambda... if fuel is 14.13 then it's still 1.0 lambda but the gauge will read 14.7 see the confusion? Is there anyway to get WB data to data logs?

When using a narrow band for fueling well it can be off as well because of the narrow band. Here's what I just spent hours and months trying to figure out.

I can run OL on my tune and lambda gauge and serial WB data steady at 1.0 Lambda.

If I go Closed loop:

Bosch O2 sensors = .97
Second set of Bocsh O2 sensors = .99
AC Delco O2 sensors = 1.02
Denso O2 senors = .99

All sensors were new when installed running same tune.

HTH!

[daniel76309]

Thank you for the additional info. I am not sure I understand completely...

It is my understanding, and I think you confirmed, that the WB reads lambda as ~14.7, regardless of fuel. Are you saying this is NOT the case with the narrow bands, i.e. that they will be "inaccurate" when using a blended fuel? So that being the case, it actually IS running a little rich and the WB is reporting accurately?

One thing I want to do before I get too far down this path is to try some ethanol-free fuel. I recently saw a sign at a gas station not too far away from my house that was advertising ethanol-free fuel. Hopefully I can still get that--it would be worth trying just to see if in fact it changes things.

The other thing is that I believe there is a way to create a "custom PID" that uses a formula to convert WB output voltage to lambda values that go in the datalog. I seem to remember seeing some disagreement on which formula to use however, so I need to look that up again and see if I can figure it out. I think it might be worth doing this early on, just to make it less confusing going forward.

[EagleMark]

Yes the wideband reads Lambda and converts it to 14.7 to 1 AFR for your viewing pleasure.

Narrow band O2 sensors adjust to Stioch = Lambda on your WB gauge.

What I was trying to say about NB O2 sensors is after the few I've tried they are close to 1.0 Lambda, some higher and some lower.

Non ethonol fuel is not going to change a thing. OK it's better fuel, you can get more HP and MPG, but it's not going to change what we are talking about.

I think you missed the biggest point, your tune Desired or Stoich AFR is still set to 14.68, so if your fuel is 14.13 then there's your fuel trims!

Having a Lambda PID for tuning is going to be your second best change, along with Stoich AFR in tune. Then just forget about AFR and live life of Lambda.

[daniel76309]

Thank you again, but now I am REALLY confused.

When you say "I think you missed the biggest point, your tune Desired or Stoich AFR is still set to 14.68, so if your fuel is 14.13 then there's your fuel trims!", are you saying that if my ECM calibration commands an AFR of 14.68:1 it will actually end up being 14.13:1 (as reported by the wideband) on fuel with 10% ethanol? If so that doesn't make sense to me because the way I see it, in order for that to happen, the NB's would have to be able to determine fuel compostion (which as I understand it, they can't).

I would THINK that theoretically (i.e. assuming perfectly accurate wideband and narrow band o2's), if my calibration calls for 14.68:1 AFR, that in closed loop operation the NB's would compensate for conditions to produce an AFR that would be read as "14.68" on the WB, and that would occur regardless of the ethanol content of the fuel. No?

Based on your findings of testing different narrow band O2's, could it be that everything is as it should be, within the + or - 2% error of the NB's?

I am going to have to mull this over...

[EagleMark]

The narrow band O2 sensor does not adjust to AFR, it adjusts to Stoich of the fuel being burned. Lambda is that reading of exhaust and 1.0 is a perfect Stoich.

So if you have E10 fuel which is 14.13 AFR = 1.0 Stoich and your tune starts the fuel calculation with 14.68? Then the narrow band O2 sensor will adjust it to Stoich with Fuel Trims! You could tune it out and get LTFT to 0, but the correct way is to start the tune properly calibrated.

You'll have to ask someone here more familiar with this PCM OS and HPTuner how to get the Stoich AFR table I saw in order...

If you tune with the NB O2 then you'll never notice they are off or can differ. But I tune Open Loop with Wide Band so I instantly notice when I go Closed Loop.

[daniel76309]

Thanks, that helps--I think I am catching on now...

Correct me if I am wrong... If we take the NB's out of the picture, i.e. run in open loop as you have been saying, the fuel injectors are going to deliver a certain volume "X" of fuel as specified by the calibration under given conditions based on MAF input, RPM, TPS, etc. That volume of fuel would theoretically be the correct volume of fuel IF it were gasoline, but in fact it is "diluted" with "up to 10% ethanol". So as a result, it is somewhat leaner than stoichometric.

So, adding the NB's to the mix, they detect the lean condition, fuel is added to correct it, hence the positive fuel trims. Correct?

But then theoretically, the WB should also read 14.68. But the fact that it is more like 14.3 (~2% error) could easily be attributed to the NB's being a bit "off".

So the NB's are seeing a lean condition, which is to be expected based on the fuel, and the LTFT's are correcting for that. However, due to NB error (presumably), it probably isn't quite as lean as the NB's are detecting, so there is a slight degree of overcorrection.

Does that all make sense?