I recently bought a 2011 GSR with 36k miles. Car was 100% stock when I bought it and from what the PO said, it was never modded. I noticed a occasional rough idle a month after I bought the car, so I replaced the fuel pump relay, changed air filter, cleaned MAF, and changed the spark plugs.

I put an ATP catless test pipe on as well as an ETS v2 extreme catback, put an AEM wideband in, and flashed the cobb stage 2 LWG map. I haven't been getting into boost and won't until I get a protune because of overboost/fueling issues with the OTS maps.

My problem is that my wideband is reading 17.xx+ at idle. My AP shows my LTFT @ idle is 12.5% (which I believe is the max allowed LTFT? correct me if I'm wrong) but my cruise stays around 14.8-15.4 and my LTFT is 5.5%.

-I've checked for vacuum leaks by spraying brake/carb clean around just about everything I could see post MAF, and by doing a boost leak test. I've zip tied the usual vac leak areas on this car. (from what I know of) Held 20psi then blew out a plug I used for the valve cover breather. I didn't hear any leaks in that test, and I don't have access to a smoke tester.
-I've checked for exhaust leaks, and found one at the doughnut gasket from the o2 housing to downpipe. Fixed the leak and still lean at idle.
-I've replaced the upstream o2 and it didn't change anything.
-I've swapped to the stock tune and it acts exactly the same.
-I've tried swapping AEM sensors with a brand new one, and it read exactly the same

Any advice as what to do/check next before I hand it over to a shop?

I can post idle and cruise logs at some point tomorrow.

Sorry if I posted this in the wrong section.
Thanks

 

lean at idle, but not at cruise = exhaust leak, likely between down pipe and test pipe. Try one of these:

 

Wouldn't an exhaust leak after the upstream o2 sensor only cause the wideband to read lean? Or does the downstream also adjust fuel trims on this car?

 

Have you checked your pump or relay? Our relays (and pumps) are JUNK!!! Just a thought...

 

after i got new injectors and a downpipe (plus tune) i was running lean...and then rich after a days worth of troubleshooting. it was definitely frustrating, but as it turned out I had a faulty front o2 sensor and one of my injectors was malfunctioning. replaced those things and got retuned, problem solved. the o2 sensors are pretty pricey, but might be worth looking into