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Discussion Starter · #1 ·
Post engine modification I was getting a P0172 (Fuel trim system Rich) code in both my original PCM as well as my new PCM. Due to my injector size upgrade it was thought that a mini AFC would be able to adjust for such a modification, as the NEW PCM did not have the new injector size programmed in it. It was found that my O2 values (by means of a Tech 2) were fluxuating anywhere's between 50-1100 MV. LTFT values were in the range of -22% at idle, and never dropped above -15% at WOT, and the STFT was around -7% at best. The Mini AFC was found to be of no help at the time, so after much research and questioning I decided to have my PCM flashed with my 'new' injector size, and see if that helped.

Just this week I installed my new PCM, and the difference in the car was night and day. I allowed the newly flashed PCM to learn for 2.5 days before I put the Tech 2 on the car to see if it had rectified my problem. My LTFT is much better now and was between -14 and -12%, and the STFT was right around 0. The computer hasn't flashed any codes so that would indicate that the Fuel Trim values are withing the parameters for the PCM to compensate (11% either rich or lean I believe), however,,,,,,,, my O2 values are still fluxuating between 50-750 MV so I am unable to trim the car to where it should be (940-960MV).

I have further details for those who require them, but for now I just thought i'd leave it at this.

I am wondering if anyone has seen such an occurance with their O2 sensor. Also wondering if I may be able to tune using the AFC and the LTFT/STFT values as opposed to the O2 values.

FYG at present:

-The O2 sensor is new.
-Rear O2 sensor is a simulated value
-MAF @ idle 3000 hz
-MAF @ idle 8.3 g/s
-Injector pulse @ idle 2.3ms
-A/F ratio 14.7:1

Thanks in advance to anyone who might be able to shed some light in order to get the "little darlin'" working where she should be.
 

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1998 GTP coupe. All stock and rust free. That will change soon.
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Is the fluctuating O2 value at WOT? The front O2 sensor will normally fluctuate between 0 and 1V during normal closed loop conditions. If it's moving around like that at WOT then there are issues.

The DTC you got was because the PCM could not lean out the system any more based on your upgraded injectors. Now it's within the allowable range for LTFT (-22% to +16.4%). I think you could still use a mini AFC v2.0 to further tune your LTFTs at idle and cruise and tune your O2s at WOT.

MG
 

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Discussion Starter · #3 ·
The O2 values were fluxuating from 50MV-750MV at idle, and pretty much the same WOT. So they are supposed to fluxuate like that at idle?

I have a Mini AFC version 1.1 ready to use. You mentioned the v2.0. Is there a differnce between the two AFC's for my intended application?

So even if my O2 values are fluxuating like this, you believe it's okay to tune using my AFC by means of LTFT?

Thanks very much
 

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1998 GTP coupe. All stock and rust free. That will change soon.
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B1S1 is supposed to oscillate at idle/cruise, but it should be fairly steady at WOT. The AFC 2.0 lets you adjust MAFF scaling in decades (i.e. 1KHz, 2KHz...through 13KHz) to make a custom curve, whereas the AFC 1.1 scales the same across the entire MAFF range. So with the 2.0 you can lean out at idle and cruise, then fatten up at WOT. The AFC 1.1 is a good tool but the AFC 2.0 allows more control.
 

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Discussion Starter · #5 ·
Great info, thanks. I'm going to try the AFC 1.1 first to see if i'm actually gaining ground with the AFC route. Once I find out that this is my solution, i'm going to consider the AFC 2.0.

Where the LTFT is still around -12 % or so, would you recommend starting at maybe 98% on the AFC??

Thanks.
 

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yes at idle the B1S1 sensor does flux. I see any where between 20-940 at idle/cruise... I see a 935mV o2 level at WOT, its steady, doesn't move as long as TPS is 100%

If you have a mini-afc that will help you greatly... remember with AFC 2.0 you tune your 1K-8K MAFF frequencies by averaging your LTFT.... Its a -# you tune AFC down 1% at 1% increments until your LFTF values are as close to 0 as possible. Same goes for your LTFT avgerages if they are +# you tune up, at 1% increments...

Best of luck tunning your car.
 

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2000Falcon said:
Great info, thanks. I'm going to try the AFC 1.1 first to see if i'm actually gaining ground with the AFC route. Once I find out that this is my solution, i'm going to consider the AFC 2.0.

Where the LTFT is still around -12 % or so, would you recommend starting at maybe 98% on the AFC??

Thanks.
No! The AFC 1.x can be used effectively for tuning the WOT O2 only. If you lean out using the AFC 1.x to get your FTs to 0, then you will also be leaning out at WOT as well.
 

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Discussion Starter · #8 · (Edited)
Dont worry, I haven't touched anything yet, I was away all weekend. I have to get the Tech 2 back on her this week and check a couple of things out anyway, and go from there. Sounds like an AFC 2 is on the books.

I really want to do the right thing here, and I'll admit i'm pretty green to the computer diagnostic sit here. I've been doing my homework, however, im taking all suggestions into consideration.

Where would YOU recommend I start with the car, the tuning, and with what? I am ready to take serious measures to get her working just right.

thanks :)
 

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Ross,

Please answer a few things for me.

- What size injectors are in the car - never mind, I see 42.5
- What size injectors have been programmed into the PCM?
- What is your AFC 1.x set to?
- What are your LTFT's at idle?
- What are your LTFT's at cruising (Foot slightly on the accelerator)?
- What are your LTFT's at WOT?

You probably do not need an AFC 2, it all depends on how drastically your LTFT's are changing from one trim cell to another.

You say that your O2's fluctuate at WOT? It's okay for them to change a little, but not drastically. With your Tech 2 scanner check the closed/open loop flag at WOT and make sure it stays in OPEN for the entire time that your TPS is over 35% - 40%.

I'm sure we can get yoiur car tuned properly.

John
 

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Discussion Starter · #10 · (Edited)
John, Thanks for the reply! I have some of those values right now, but i'm just on my way out the door to put the car on the "magic box", so i'll get back to you later today with all of the info. FYI, I dont have the AFC 1.0 connected at present as i've been waiting for the "official" go-ahead. Thanks again!

By the way,,,,,,,,,,,,,,,, VERY NICE CAR! :cool:
 

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Thanks :)

So the car is running without the AFC right now? It should be able to with the injectors programmed in. One more question.

What is the highest MAF frequency that youever see at WOT?

John
 

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Discussion Starter · #12 ·
johnt said:
Thanks :)

So the car is running without the AFC right now? It should be able to with the injectors programmed in. One more question.

What is the highest MAF frequency that youever see at WOT?

John

John,

Yes, the LTFT values in my original thread (-14 to -12) are values at highway cruising. The car is running fine with no AFC. I will get the MAF values for you today along with the other requested info. Have a good one!

Ross :)
 

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Discussion Starter · #13 ·
johnt said:
Ross,

Please answer a few things for me.

- What size injectors are in the car - never mind, I see 42.5
- What size injectors have been programmed into the PCM?
- What is your AFC 1.x set to?
- What are your LTFT's at idle?
- What are your LTFT's at cruising (Foot slightly on the accelerator)?
- What are your LTFT's at WOT?

You probably do not need an AFC 2, it all depends on how drastically your LTFT's are changing from one trim cell to another.

You say that your O2's fluctuate at WOT? It's okay for them to change a little, but not drastically. With your Tech 2 scanner check the closed/open loop flag at WOT and make sure it stays in OPEN for the entire time that your TPS is over 35% - 40%.

I'm sure we can get yoiur car tuned properly.

John

John: Here's what I've got for ya:

- Injector size 42.5 lb.
- 42.5 lb injectors programmed into my Stage 2 PCM.
- No AFC connected at this time.
- LTFT's at idle -14 to -12 %.
- LTFT's crusing -12% thereabouts
- LTFT's @ WOT -9% thereabouts.
- O2 readings @ WOT 915 to 930mv (only slight fluxuation as shown, 15mv)
- The "closed/open loop" flag was "OPEN" for the time throttle position between 40-100%.
- MAF @ WOT between 270.12 to 311.6 g/s.
- MAF frequency @ WOT between 10000 and 10111 Hz.
- TPS at WOT was 100%.

Anything else, just let me know. Thanks again.

Ross :)
 

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Ross,

Other than Your LTFT values being a little more on the negative side of neutral, everything seems fine. I like the WOT O2 readings of 915 to 930. A little bit of deviation like you're seeing is perfectly normal. I only asked about the status of the open/closed loop flag because it wasn't clear how much your O2s were fluctuating by while at WOT. OPEN means that the PCM is working off of internal tables and is in PE mode where it's targetting a much richer AFR. At high TPS values, that's what it should be doing so you're ok. Your MAF frequency looks about right for a 3.4 pulley. In short, it looks like your problem is solved. :)

Let the car learn a little more, as the LTFTs will still likely change for another day or two. It doesn't harm anything if they stay negative, but if you want them closer to zero we can do that by editing your IFR table in the PCM. In other words, we could tell the PCM that you have 40 lb injectors as an example.

As of now, I don't see any obvious problems. If you want, take a scan in a week or so and send me the file.

One more question I forgot to ask you. What injector pulse widths do you see at WOT in high RPM's?

John
 

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Discussion Starter · #15 ·
johnt said:
Ross,


Let the car learn a little more, as the LTFTs will still likely change for another day or two. It doesn't harm anything if they stay negative, but if you want them closer to zero we can do that by editing your IFR table in the PCM. In other words, we could tell the PCM that you have 40 lb injectors as an example.

As of now, I don't see any obvious problems. If you want, take a scan in a week or so and send me the file.

One more question I forgot to ask you. What injector pulse widths do you see at WOT in high RPM's?

John
John:

I am very glad to hear such positive news regarding my prolonged agony and questions.

I didn't get the injector pulse widths at WOT in high RPM's, however they are 2.3 m/s @ idle. I know that doesn't mean much with regard to what you're looking for, but it's a number I had anyway.

Just for my own curiousity (not really knowing exactly how the AFC's work), could the AFC be used to bring the LTFT closer to 0%? I understand fully what you said about editing the IFR table in the PCM. What sort of performance gain could I possibly expect by getting the LTFT values from -10% (where they are now) down to zero? Also, regarding the O2 values, is it "the higher the value, the richer the mixture", or is it the opposite. I was told originally that the O2 values should be preferably between 940-960 mv. I'm just curious as to what sort of difference, if any, we would see with a value change such as the one mentioned.

Thanks again for your time John, my day has been made! :)

Ross
 

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Ross,

There would be no performance or drivability improvement in getting the LTFTs closer to zero. At least, not that I've ever experienced. The only advantage is that being closer to zero there would be more room for the PCM to make adjustments, if it had to, without hitting one of the rails. You could use the AFC to bring them closer to zero, but to tell you the truth, I'm not fond of spoofing the MAF sensor as a roundabout way of adjusting fueling. I feel that if you want to adjust the fuel, then you should just adjust the fueling in the PCM. That's a better solution rather than bringing something else out of balance intentionally to balance the fuel out. If that makes sense. Besides, The MAF is one of the two most important sensors on the car (the other being the O2) and I'd rather not introduce another potential failure point in series between that and the PCM. I think the AFC would be a better solution if you were to be overrunning your MAF table and needed to scale the frequency down a bit, but that doesn't seem to be an issue here.

With O2s, the higher the value means a richer mixture... usually. :) To get a better understanding of what I mean by that check out another post I made:
http://www.3800tech.com/forum/showthread.php?t=848
where I talked about the narrowband O2 sensor's sensitivity to exhaust gas temperatures.

To answer your other question, I think 940-960 is a little too rich for most cars. I am currently running the third motor in my car (blew the first, upgraded the second stock motor to a third forged one) and all my motors were happiest between 920 and 935.

Keep monitoring things and see if you can get a scan once your trims have settled down a bit. Let me know if you experience anything that doesn't look quite right.

John
 

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Discussion Starter · #17 ·
All VERY interesting John (including your thread about the O2 sensors), thank you. I agree with you in regard to the "non-necessary" MAF sensor tricking. Looks like my O2 values are directly in-line with what you've found 'works', so that's great!

What would you consider too lean of an O2 value? I would guess around 900 or so? This is motor #2 for me, so i'm hoping that there's not a #3 in my future. :rolleyes:

Since my rebuild, i've been wanting to put the car on a dyno, but haven't because I wanted to make sure she was running where she should be. That being said, I would imagine she'll see one soon. Seeing as at some point in your modding extravaganza you would have seen a motor done up like mine, do you have any estimates as to what it may be running for HP at present? I'd just like to go in with an idea as to what I might be able to expect.

Once again John, thanks! :)

I'll be sure to keep my eyes on the Trims and let you know if I find any critters.

Ross
 

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John, this is really good information. I agree that the AFC introduces a failure point between the MAF and the PCM, but for most mildly modded folks the AFC works well. It really shows how robust the PCM is when tuning for performance and economy.

Ross, you want your O2 value to be as lean as possible without knock. I agree that .910 - .920 is a godd target but as John states in his other thread the narrow band O2 sensor is really just a reference point. manufacturing variations in sensors and car-to-car variations make stating an absolute stock O2 sensor voltage impossible. The answer is that you have to scan your car a lot and tune for your combination with each and every change.
 
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