I have been messing with some odd trim issues for awhile, and I made some observations today based on my old logs. On decel (high vacuum) my dynamic cylinder air seems peculiarly low exactly where my trims go positive. Also, on the low end my trims seem to randomly go rich (way way way positive) at places where dynamic cylinder air may be too high for that level of airflow. Is this a VE table issue? My trims were averaging + or - a few away from zero before I put my MAF back on, but for some reason I suspect a VE issue. It is the only thing that makes sense. Is the calculated (and logged) dynamic cylinder air actually biased based on the dynamic airflow calculations (meaning the calculated dynamic cylinder air will be based more on VE based on the dynamic airflow calculations, and at other times based more on MAF airflow)? I am open to any thoughts. Something else I noticed that led me in this direction was that my dynamic cylinder air moved with MAP, while the MAF airflow stayed steady.

 

I'm going to quietly sit in the corner and listen to this one, and hopefully learn more. Lots of good things can become of this thread if the attention gets to the right people. I am going to mess with my IAC vs. Eff Airflow table tomorrow trying to get rid of the last bit of creeping Cruise Control and now and then surge.

Charlie

 

Could you go into more detail about why you suspect this to be a VE table issue? It would seem to me (on the surface at least) that if your trims were good before the MAF was plugged back in, that you could pretty much eliminate the VE table as the culprit. I'm not quite following you on why you suspect the VE table.

You bring up a good question about dynamic cylinder air. I have a couple of ideas about this, but I'm going to sit on them overnight until I can try some things out on my car tomorrow to prove/disprove what I'm thinking about.

 

Well I ended up on this track because most of my problems were at low airflow, either closed throttle or light throttle, low rpm. A lot of where the VE calculation (I think) should start to play a larger role. I noticed that in the areas I typically see problems the dynamic cylinder air is following MAP, but I was expecting it to follow the MAF airflow since that is the primary method of measuring airflow when it is plugged in. This leads me to believe that there is some issue being caused by VE bias in those areas. I think. That is why I made this thread, LOL!

 

what do your ftc boundaries look like

 

Oh, how do you check FTC Boundaries?

 

Fuel Control----> Open & Closed Loop ---> LTFT Boudaries

 

Dynamic Cylinder Air is per revolution, so closer to MAP
(density) while MAF airflow embodies the RPM (speed)
as well so would diverge from CylAir, by that dimension.

I've seen remarks that CylAir also has some smoothing
done to it. I do not know the entire algorithm (or even
much at all).

 

Well I would be interested in finding out more. I set my FTC boundaries back to stock. They aren't the problem. My old changes did not make any difference either.

 

I spent a little while this morning logging Mass Air Flow, MAP, Dynamic Airflow and Dynamic Cylinder Air so I could chart it out and see their relationships under throttle transitions. The results really demonstrate how innacurate the MAF is during throttle transitions, and why the SD calc has to be used to "smooth" the MAF data. It also gave a good graphical representation of this "smoothing" at work. (See the attached pic)

Looking at the DA waveform, we can see how it ramps up to peak airflow, and ramps back down to idle airflow, with a gradual slope during the throttle transitions. Conversely, the MAF airmass waveform jumps up to peak airflow and drops back down to idle airflow with almost no slope at all. It's almost like turning a switch on and off. Airflow is of course going to gradually increase to peak and gradually decrease to idle like shown by the DA waveform, so the MAF waveform is showing us graphically the inability of the MAF to follow throttle transitions with an acceptable degree of accuracy. Looking at the peaks of both waveforms, we can also see that the MAF airmass peaks sooner than the DA.

Now, if you look at the DCA waveform, you can see how the PCM is smoothing/blending the MAF airmass reading with the DA calc. The DCA waveform is showing smooth transitions, like the DA waveform. If you reference the DCA peak with the others, you will find that DCA is peaking in between the MAF airmass and DCA peaks, although it peaks (in most cases) much closer to the MAF airmass peak. This would seem to show that the PCM is relying most on the MAF airmass reading, which it "smooths" using the DA calc during throttle transitions which the MAF just can't follow accurately. This is of course not a new concept. We're just seeing it in action, so to speak.

I observed during my logging that DCA followed the MAF airmass reading completely during steady airflow, i.e. at idle, or cruising. During throttle transitions, it still followed the MAF mostly, but DCA was "smoothed" to compensate for the MAF's inability the accurately follow the changing airflow. I find that the MAF airmass reading is still the most telling factor. Are your fueling issues happening at constant throttle, or rapidly changing throttle transitions? The MAF airmass reading is only filtered when the PCM detects a throttle transition, and it is filtered to the extent dictated by what MAP vs. RPM zone the engine is currently operating in.

My stance on the matter is that your VE table is correct. Your trims during SD operation prove this. The Dynamic Airflow calc is correct by definition (as long as your inputs are good), so you can rule that out too (unless you inputs are bad ). So, that leaves the MAF (most likely I think) or the actual filtering applied to the MAF signal by the PCM, as dictated by the data entered into the Dynamic Airflow tab in HPTuner. Personally, I think the engineers at GM probably did a pretty good job at setting up that particular dynamic filter, another reason why I think it's the MAF.

Could you post up the logs that show these trim problems you've been having? Or email them to bluetorp@yahoo.com ? It would help me understand a little better the problems you're having.

 

Ok, here is the problem that baffles me the most, and then maybe you can shed some light on it. My trims go positive on decel (+10-15 or more depending on how long you decel, not so bad with DFCO enabled), and while I am at steady throttle my trims will go way negative (-40+), steady MAP and MAF airflow, and the frequency is not jumping around. But the O2s are definitely stuck rich, and then all of a sudden they stabilize. I have trouble justifying that it is a mechanical issue because both banks do the exact same thing at the same time, so I am positive it is somewhere in the tune. Attached is a link to an excellent log. Let me know what you think.
http://users.adelphia.net/~someoneelse/14APR05.hpl

 

Holy crap. Now I see what you're talking about!

Since it's happening at constant throttle, you can eliminate the DA calcs because they are not being applied to the MAF signal since no throttle transient is occuring. The MAF signal looks good, nothing crazy going on there.

Your STFT's are diving down to -40 because your O2's are locking super high at around .900. The real question is why they are doing this. Like you said, it's probably not the O2's themselves since they are both being affected in the same way. Another thing that I notice is that your O2's never go into a good pattern (sine wave). They are showing a very jagged and irregular waveform. Have you modified anything that you can think of that might be affecting your O2 operation?

 

No. I have tested things, but my O2 settings are stock. Interesting isn't it? It only happens with the MAF plugged in. It never happens in SD. I tested again today.

 

Hmmm...

Do you have a log that shows what DA and DCA are doing when your trims take a **** like that?

Also, do your O2's drop into a good sine wave pattern at cruise when running SD, or do they look the same as they do in the log?

 

I just logged them both after un-failing my MAF, same thing as always, MAF working, weird trims, SD, trims are fine. DA and DCA match almost perfectly straight across, there are a few times during transients where they don't for a few ms, but even then they are still really really close. I have never seen my O2s settle into a smooth like at any time except idle. I can't find an airflow related reason that really explains what is happening. If it were bad O2 sensors or injectors or valves, or anything it would not be exactly the same on both banks, and it would not trim just fine in SD mode. I don't get it.

 

I just realized that DCA was on the log you posted earlier, too.

I can see no reason for the O2's to be reading like that. MAF airmass and DCA are steady, as well as injector pw, and everything else (MAP, TPS, etc.) as well. What's really strange is that it only does it with the MAF plugged in. Hmmmm...

Those O2 waveforms just look crazy to me, aside from locking at .900. I'm attaching a pic of what mine look like at cruise. Correctly operating O2's should look something like this.

 

They O2 readings look a little wacky to me. But if you want a real treat you should see my O2s locked at over 1000mv in open loop. Only happened once. I didn't even know it was possible....but anyways...

I have been pondering this issue for awhile. The only thing I can think of that could cause my trims to go this rich without an increase in injector pulse width is a malfunctioning EVAP. You wouldn't happen to know if EVAP functions differently in the event of a MAF or similar failure? Tomorrow I am going to log a run with EVAP forced off and see what happens...

 

Definitely something to test, but I'm not aware of any difference in operation as far as EVAP is concerned between normal and MAF failure operation. But then again, there's a ton of stuff I'm not aware off! At this point I'd be testing anything and everything!

How consistent is this problem when running the MAF? Is it at all possible that it may just be coincidence that it's only popped up during MAF operation and not SD?

 

No, it is definitely not a coincidence. I have MANY logs.

 

Have you tried another stock MAF?

Also have you taken note of what FTC you are in when the the problem is occuring?