Hi all, brand new here and fairly new owner of a 2008 LS3. I'm having an issue where my car runs lean until closed loop, or when the O2's heat up and start correcting AFR then it improves but is still wonky fluctuating between 14.7 and about 16.5. Car has O2 CEL's removed by tune. I am suspecting vacuum leak after the MAF in the intake boot, throttle body gasket or intake manifold gasket. Plan on using a smoke machine or carb cleaner to take a look, may also smoke the EVAP system and pressure test at fuel rail to be thorough. Car has new fuel pump relay, plugs, wires, O2's replaced twice, fuel pump and gaskets have been replaced as well. I figured I would post here for any other suggestions. Thanks!
Mods:
Vararam intake
Stage 3 TSP cam
ARH long tubes
Kooks X Pipe
NPP mufflers
500whp 476tq

 

Maybe your air filter is allowing too much air to flow.

 

Like a loose filter? Being before the MAF wouldn't that throw a CEL for lean condition? Regardless I'll take a look not hard.

 

I wasn't thinking of a leak, but rather that your aftermarket air filter may be allowing more air flow than your custom tune is able to deal with.

 

During a 'cold start" without the O2's providing feedback to the ECM the engine relies on a few inputs being ECT, IAT, MAP, TP, and MAF...I'm assuming you have a scan tool so when the car sits overnight compare the ECT and IAT to the outside air temp BEFORE starting the car with the scan tool...all 3 should be fairly close together...also look at your BARO PID...if you are near sea level it should be around 29.9 inHg...MAP psi should be around 14.6 and MAP voltage around 4.85 or so...if your scan tool has the ability to look at Generic OBD2...it has a check engine light symbol and you don't have to enter the car year etc...go into that...there is something called "substituted values" in where what you see on your scan tool is not what the ECM is seeing...in Generic OBD2 those values like IAT, ECT can't be substituted...this is a Federal mandate blah, blah, blah to whatever SAE rule governs this...if your ECT is reading 120 degrees and it's a cold engine the injector pulse width will be small giving you a lean condition...same for the IAT...once the engine goes into closed loop and if your long and short term fuel trims are elevated (over 10%) raise the RPM to 3000 if you suspect a vacuum leak...if you have a vacuum leak your Short Terms will quickly decrease and your Long Terms will gradually decrease after a few minutes...the EVAP system is not connected in any way to what you are seeing...the only thing would be a stuck open purge valve in which case the car will run rich because it is sucking in fuel vapors from the EVAP cannister...also your MAF should be reading roughly 6.2 grams/second at idle with the engine at normal operating temp...that number is the liter displacement of the engine and this is a "rule of thumb" number.

 

You are new to the car correct? I would be suspicious the cold start part of the tune is off if what C5 Diag check's out are their are no faults.

Their are more bad tuners than good, you have left out a lot critical information for help other than guessing.

 

Did you look at the open EQ values in the tune? It may be tuned to purposefully run lean in cold open loop. I do this for all my vehicles because it cuts down on carbon deposits and keeps the plugs clean while the engine is still cold. You can't (shouldn't) run a cold engine hard while its warming up anyways so just let it put-put around for a bit while it gets warm is fine.

The only reason they run em rich when cold is either kick start catalyst and appease the driver throttle response. But that is with the OEM camshaft. Large cam is going to drive a bit rough while cold at low speeds (cold air, low velocity air) and inject all fuel when cold (OEM ecu) to the closed intake valve which will light the catalyst thanks to overlap anyways. And the cadence misfires of a large cam especially while cold contribute to a leaner-than-realist wideband reading as well.

I would
A. pressure test the plumbing for leaks just in case
B. make sure the sensors are reading right
C. investigate the tune file and adjust as desired

 

Also, a leaking exhaust manifold will also cause a lean condition regardless of engine temp. But from reading the above posts, and tuning out O2 CEL's sounds like this was done on purpose. Are both banks lean?