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!