Sorry this may be a bit long. Need to see if anyone has had this issue. ive searched but dont see the same thing. Back in early DEC 2020 my 2013 G37 IPL Vert started to have an issue with engine performance. My car has a Stillen Catback exhaust, Gen 3 CAI, Z1 Hi-flo Cats and Z1 tuned it for me. The car has about 125K miles and at ~100K i had the throttle bodies cleaned, new plugs and general service. So in mid Dec it started to have an issue when acceletating or if RPMs got over 3K. I could drive sanely and it will keep gaining speed but if i accelerate hard or quick it starts to stutter. I took it to the dealer and at first they said sounded like the tune bad??? So i talked to John at Z1 and he said tune doesnt just go back may be coils. But i couldnt get an appointment there until May 2021 for them to even look at it. So back to dealer in late Dec and it is still there. So they put the OEM tune back on and still happens, they changed some coils and still happens, they said has good gas pressure. Im affraid either i cant afford the storage fees or they cant figure what is up. I hope a Tier 3 guy from the Land of the rising Sun is coming to check it out. So not coils, not MAF, not tune has anyone ever heard of this or had this diagnosed and fixed? Really want my car back. ive searched all over the internet and nothing comes up that they havnt checked. Please any help is appreciated.

 

Sounds exactly like a bad coil. Trouble is which one is it? Regardless its time for a second opinion from someone who knows what theyre doing or cares enough, obviously the dealer is incapable of either or both. I would get a code reader and try it yourself. If you can diag the problem you will have a better chance not getting jerked around by mechanics, especially the dealer. Do you still have a warranty with them or something? If not there really is no reason to go there anymore. Get your car back. See if you can pull up any codes. Get a new coil and replace them one by one until you find the bad one. Could be more than one tho. Take matters into your own hands.

Edit I have some coils if you want to meet me up at bethlehem publix sometime I can give it to you. I also have a code reader.

 

Did you have a flame map with the tune?

 

Most of these issues that folks complain about seem to wrap around exhaust leaks and it could be the reason why most overlook this including myself. Exhaust leaks will affect your o2 readings and these leaks can be contributed to tune being that most tuned take advantage of these. Your exhaust will see these results and will start to show it's age. Any and I mean any leaks will throw your airfuels off. Any leaks will cause a lean reading forcing the engine to add additional fuel, more than needed which will throw everything off.
Whatever your problem is it's mechanical.