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Thread: Closed Loop / part throttle issues

  1. #1
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    Jan 2012
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    Closed Loop / part throttle issues

    Looking for some guidance or clues to tune-out a driveability issue I’m running into. I live in CA and am subject to the bi-annual smog inspection here (still includes tailpipe sniffer). First, the setup:

    Hardware:
    08 z51 / Ls3 / 6sp
    Halltech CAI
    FAST LSXR 102
    223/232 115+3 cam
    LS7 Manifolds / H-pipe with Catalytic converters
    New plugs (old ones were gapped fine and showed normal wear), recently cleaned MAF, TB. No vac leaks

    Software:
    Tuned by a reputable local shop about 2 yrs ago
    Running closed loop with MAF above 1300 rpm

    After the cam was installed, I disabled LTFTs. Everything was cool for about a year and it passed CA smog with no issues.

    About a year later, I started noticing a rich exhaust smell along with a sulphur smell when warmed up and driving around under normal part-throttle conditions. Also getting black soot on the rear bumper. I thought the Cats were bad so I swapped in another pair of low mileage Z06/LS7 ones and had exactly the same problem from the get-go (scratch that theory).

    Then I did what I should have done in the first place and did a bunch of VVE and MAF logging (using the standard procedures all over the forum: MAF/COT/DFCO/CFCO disable to nail down the VVE table and then full RPM MAF enable with COT/DFCO/CFCO disable to nail down the MAF table). I don’t have a WB so logging was done using STFT histograms. I just focused on the normal driving + idle areas and used gentle throttle movements and kept IAT between 70 and 90*F (all by the book - I think)

    After a few iterations, both MAF and VVE now give STFTs within +/-2%. Great! Right? Well, sort of. At idle, the rich/sulphur smell is pretty much gone but I’m still getting it under normal driving. In addition my city MPG is now terrible – was 17 before cam and 15 right after cam install and now its ~12. Highway mpg is still in the 25 area. Car also feels sluggish at light throttle (It used to feel better). In summary, my nose and gas tank are saying “rich” part throttle conditions but STFTs histograms are around flat.

    I’ve heard of “false-lean” conditions at low RPM with big cams but I have a very mild cam and the problem is in the 1200-2500rpm range. Idle seems ok.

    Any tips or theories before I go on my wild goose chase? My thought process is:
    1) Possible CAI + cam related MAF issues at low flow rates -- Move MAF enable up to 2500rpm and see what happens
    2) If #1 dosent work, lower the MAF transfer function in areas corresponding to normal city acceleration and see what happens.
    3) Play around with ignition timing. This seems like a longshot considering the engine is so mild and im running stock spark tables.

    Goal is to get the mixture right at part throttle so I improve mpg and hopefully pass smog and not smell like I am towing a crate of rotten eggs! Feedback appreciated!

  2. #2
    Tuner blkscooby's Avatar
    Join Date
    Aug 2010
    Location
    Mesa, AZ
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    81
    Did you check the idle air table? It needs to be tuned for a larger cam. How does the car idle? smooth or lumpy?
    09 G8 GT- 224/234 114 cam, 3200 stall, 3.45 gears, Vararam, 1 7/8" headers, 3" exhaust, HSW nitrous

  3. #3
    Potential Tuner
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    Jan 2012
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    Idle is smooth as silk. can barely tell there's a cam. I adjusted the min airflow table a while also to get rid of some mild trailer-hitching I had at 1500 rpm.

  4. #4
    Potential Tuner
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    Jan 2012
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    so it turns out after a bunch of trouble shooting (looking for vacuum leaks, and exhaust leaks around the 02) i tried retarding the idle timing on a lark. i took it down from 18* to 10* and it worked!! -The idle smell is gone. the STFTs still read +/- 2%. This low timing is not near the peak MBT that people talk about for optimizing idle (you can tell because there is less vacuum at idle with the lower timing) but my theory is the lower spark advance is increasing EGT and heating up the cats better and allowing them to scrub whatever NOX or HC was coming by. it now passes tailpipe sniffer. Anyways - feel free to poke holes in my hypothesis but I wanted to share in case there were others with smog trouble on otherwise well tuned and mild cars.

  5. #5
    Tuning Addict
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    Jan 2015
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    Franklin, NC / Gainesville, Ga
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    What did your fuel economy do? If that's all you changed, you might even be able to go back in and set your idle mbt timing where it was and then take whatever amount back out of your idle advance table as long as you still have somewhere in the 10 to 12 degree possible advancement still left. Otherwise, if it works, it works...
    2010 Vette Stock Bottom LS3 - LS2 APS Twin Turbo Kit, Trick Flow Heads and Custom Cam - 12psi - 714rwhp and 820rwtq / 100hp Nitrous Shot starting at 3000 rpms - 948rwhp and 1044rwtq still on 93
    2011 Vette Cam Only Internal Mod in stock LS3 -- YSI @ 18psi - 811rwhp on 93 / 926rwhp on E60 & 1008rwhp with a 50 shot of nitrous all through a 6L80

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