Can't seem to get rid of surging idle issue - VVE tuning

[adunphy]

2007 Silverado 4.8L (E38/T42); 72mm Turbo, turbo cam, 88lb injectors, walbro 450lph in tank, 4L80E

I've been running in MAF only mode for over a year. I made 518HP and 551lb-ft at the wheels. Power tuning was a breeze with the MAF. As I ran out of Hz on the MAF I decided to increase my intake tubing to 4" and switch over to a card MAF. At very low airflow the MAF is OK at best so idle and cracking the throttle had room for improvement. Tuning the MAF for the new larger tubing was fairly straight forward.

In an effort to smooth out the low throttle response and idle a bit, I upgraded to the COS 2-bar OS. I now failed the MAF to tune the VVE. With a combination of the GMVE PID, and the formula to calculate GMVE using my WB02 for error correction, I was able to rough in a VVE table. Sure its ugly but it got me started. After about a month I am still chasing my tail with a surging idle issue. I've messed with everything from the PI (proportional, integral) controller to the min idle air to no avail. By accident, I set the low idle areas of the table pig rich and left it in open loop and to my surprise the idle was smooth as silk ~12.5:1 AFR. The moment I start multiplying by half % correction values back in the idle area, the surging starts again. Even if I simply use bi-directional to disable Adaptive Idle spark, surging starts. Seems no matter what I do, the closer I get to the correct values for VVE, the idle starts surging like crazy even in OL.

I am honestly at a loss as to where to go in the tune and am starting to wonder if its a mechanical issue (bad Throttle body, vacuum leak, etc) but I seriously doubt it. Who knows, anything is possible. Any suggestions on how I can baseline the idle?

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experiment_v0.hpt
17-01-16 18-15-17.hpl

[brandon6.0]

u might as well do away with the maf and go true speed density at this point. Or at least use the ve table only in the idle area and use maf above idle. I was having the same issues as u when I swapped over to a 4" tube and ls3 maf. I'm na cam only ls2. the card mafs move around a lot in the lower areas that affects idle a lot. Very hard to get a good stable idle with them in my opinion. They read a lot lower in the HZ area as well that a stock maf.

[schpenxel]

Same issue when I went to a card style MAF and 4" tube. I would try SD only and see what happens.

[adunphy]

I am running SD only at present and this is actually when the issue becomes worse for me. If I could get it working in SD only, I'm open to that for the long run. Its no ideal based on how the E38 is designed with dynamic air, but I could live with it.

Last night I read the entire thread on idle tuning by BigMike42 and it made me wonder if I spent enough time on the PI stuff. I also failed to mention that my tune is scaled 50% due to the large injectors and the hard limit in the ECU. While it sounds easy to just half everything airflow related, its hard to get my brain around halving the airflow correction as well. Sometimes it works like you'd expect but sometimes I cant make sense of the end result. Wish I had a flow chart of how all the airflow (specifically idle) are referenced in the code but I guess that is the programmer in me talking.

[schpenxel]

If it were mine I'd probably start over and use the double stoich AFR / half injector flow rate method and see if it's any different. I had some weird idle issues on mine in the beginning when I tried doing the half everything airflow related method. I never figured out why

I'll try to check out log/tune more later, don't have enough free time right this minute

Also, add VE airflow and idle adapt advance to your channel list

[GHuggins]

Most likely bad injector data. Had this same problem because I plugged in the wrong data for a design changed injector.

[adunphy]

Good suggestion and that is what I'd look for right off the bat, however, the injectors are infinity from "TheTuningSchool" who boast their injector data is some of the most accurate on the market. I even had Bob (owner) himself check the tune to ensure proper data and scaling. (this was after burning up my first transmission due to the delivered torque pid being off from the scaling) :-(

I am convinced the idle air and adaptive idle is way off. Going to try following the idle tuning sticky and see where that lands me. When I stumbled on that thread last night the 409 second timer bit got me thinking. If I take a drive and come to a stop and throw it in park...idles smoothly. Sit there for 5+ mins and then it will slowly start to hunt again. I am currently of the opinion that adaptive idle is plaguing me (albeit not configured correctly). I have the VVE table pig rich at idle right now which seems to help alot but clearly is not the answer. I'll post back what I find after re-working the idle.

[GHuggins]

Just looked at the log - timing, throttle and fuel are all up and down - I bet when you put it into closed loop it really surges following O2 swings... Dial back throttle and timing corrections then smooth out fueling more and see if it makes any difference...

[adunphy]

Last night I did some more testing and now I'm really confused...

Basically what I did was disable all idle correction stuff in the PI controller. I fixed my throttle plate at 20.0 TPS. It flutters a touch from 20.0 - 20.4 but hardly enough to worry about. (Not sure if thats anything to worry about). Anyway, I also set Hi/Low Octane and idle timing tables to 22 across the board. Started her up and it still hunted/surged like crazy. even if I try to feather it I gotta get to about 1200 RPM before it will smooth out somewhat. From there I started creeping up my fixed TPS% to see what would happen. Once I got to about 25% TPS and 1200 RPM idle, finally no more surging. The problem is that the cam is very mild. Its an ISKY tripple 12 custom grind (212/212/112).

After about an hour I finally flashed the tune I had previous to last nights testing and even that was hunting like crazy...huh??? How does this make sense. To further confuse matters, I took it for a ride and came to a stop and it was smooth as silk....rich but smooth. I did observe the adaptive spark was at work to achieve this.

Could my fueling be that far off in my VVE causing these massive fluctuations? Keep in mind I did start with all zeros in my VVE tables and used the following to rough the table in:

Pseudo-code:
(InjectorPW_B1[ms] + InjectorPW_B2[ms]) / 2000 * InjectorFlowRate[g/s] * WideBandO2Lambda * CommandedAFR * CommandedEQRatio * (273.15 + MAT[C]) * 1000 / MAP[kPa]

HPTuners 3.4 MATHS:
([50151.254]+[50152.254])/2000*[6210.71]*([50120]/14.7)*[50121]*[50118.239]*(273.15+[2126.241])*1000/[50030.91]

edit: I should mention that I have a dynojet wideband set at the factory to report lambda for gasoline at 14.7:1 hence why I do this: ([50120]/14.7)

[GHuggins]

Yes if SD is off or even if there are sudden drops in the neighboring cells it can cause surging. I don't know if that formula is correct. It looks screwy just looking at it. I know the air mass formula is close if you want to use it instead. It should wind up about 10 percent too rich with it.

[adunphy]

With MAF disabled or do I need I need to enable it and include in my maths? I guess I'm just not seeing how you can derive dynamic air by using dynamic air itself in the equation? As far as I can tell even cyl airmass is derived from dynamic air.

I just applied this to my Maths and ran my log through the scanner. The VVE numbers come out nearly the same as with the formula I posted in post #9 (without correction of course since there is no WB02 feedback in this formula)
GMVE based on Cyl Air (? I think this is right?)
Cylinder Airmass[g] * (273.15 + IAT[C]) * 1000 / MAP[kPa]

[adunphy]

After many wash, rinse, repeat cycles of fine tuning Speed Density with my VVE, I've concluded that my idle surging was in fact caused by my airflow model being way off. Once I got that much closer to where it needed to be, I was able to do some more tuning of min idle airflow, adaptives, and base spark to get a very smooth idle and off-idle result.

Here are the observations I made in case anyone else can benefit from my findings:

1. Use a second Wideband to sanity check your findings as well as ensure bank_1-to-bank_2 consistency. Location about 6" downstream of stock manifold in the static diameter pipe location.
2. Initially hand-smoothing of the table was necessary. Also, I found that setting the Cell Hits Required: filtering to a high value was ideal whenever possible (>=1000 if possible). Spending alot of time in as many cells as you can is paramount. You'd be amazed at how different the end result of averaging 100 cell hits versus 1000 cell hits can be. Keep in mind thats all that the histogram is doing so make sure to give it enough data so that outliers essentially get filtered out.
3. After flashing in the tune, take a ride and throw your first log away. I have seen mention in many other posts that a WOT pull is needed but all I can say is that it took me almost a month to figure out just how critical this is. I was tuning the low areas of the table in circles for a month before I realized this. I do not have a dyno so I was doing this on the street which also elongates the process, but it can be done.
4. Use the bi-directional controls in the VCM Scanner to lower idle into cells you don't normally reach. Also use the TCC control to lock up the converter at low RPM to lug the engine and reach more cells that you normally wouldn't. In this case the converter stall speed is 3200 so this worked great.
5. Initially, I did not care about the surrounding cell areas but after looking at the raw HPTuners log in excel and plotting out transitions of the throttle I could see these cells were impacting AFR during throttle transitions. In some cases I found going slightly rich (5%) in those border areas helped with throttle response. Transients is probably a better place to do further tuning for tip-in/out but both methods help.
6. Patience and persistence are needed. ;-)

I'd like to extend a thanks to GHuggins for his help and great pointers given. Thanks also to the others who posted with helpful suggestions.

[adunphy]

Just another small update in case anyone else can benefit from it.

I came across a few air straightener threads using the saxonpc honeycomb device. I got the 8:1 and installed about 5 inches ahead of my card MAF...All I can say is WOW! This thing works incredibly well. Its just amazing how much my idle smoothed out. I can now idle with a cam down to 600 without surging if I want with MAF only tune. I had no idea what sort of havoc the turbulent air was causing for me.

...and by the way my card MAF location has almost 12" of straight 4" smooth stainless tubing before and 6" after so I've found even that is not enough to straighten the air at least in this case.