Hello guys, first time posting here. I’ve had this 2009 G37s 7AT coupe for about 3 years now with 268k KMs, it has stillen GEN3 long tube intakes, HFC and a full magnaflow catback with an electronic cutout right before the muffler. everything was good until I installed intakes. Right after the install my car started misfiring lightly at idle after coming off the highway. Eventually I blew my torque converter(because of my erratic driving) and did a tranny swap but the misfire was still there and had gotten worse. The misfire is intermittent and very unpredictable, sometimes the car runs perfect and sometimes it doesn’t and when it does I take advantage of it lol. I checked for vacuum leaks and could find any, I am keeping an eye out on my fuel trims with an OBD2 reader using OBD fusion on my iPad and they’re more towards the high teens. I continued driving with it until two days ago I started hearing a rattling noise coming from my CATs, after flooring it a few times, the noise got worse and eventually pieces of the CATs was flying out of the exhaust, this is accompanied with black smoke and misfiring as-well, right while all this is happening, I start hearing a knocking sound coming from the engine which I believe is the VVEL stuck open, floored it again and the knocking went away ( had that happen a couple times before )but the misfire is still there.
Im also not sure if I ripped the intake plenum with the vacuum screw in the middle by tightening it too hard, but I’m looking forward to getting the Z1 upgraded plenum.
Ive gotten new NGK ruthenium spark plugs that I haven’t installed yet and bought some MAF and throttle body cleaners. This misfire is driving me nuts and I would like some advice from people that had misfires with their G37 it a similar story as mine.

 

We can offer no real advice until you have the codes read. Your check engine light is on, right? You should get one for random misfire, maybe two if misfiring on both sides. You should also get at least one for the VVEL solenoid. Post O2 sensor should also throw a code if your cats are falling apart.

It could be you need a tune, the exhaust or intake by itself can be handled by the ECU, but both together are often more than the fuel maps can handle, making the car run lean.

Could be you forgot to plug in one of the MAF connectors.

Is the possible plenum rip after the MAFs? That would cause a lean misfire.

You should stop driving the car until you have the codes read. Any autoparts store can do that, usually for free.

 

I do have an OBD reader and its only reading code P0420 ( Catalyst efficiency below threshold)
I figured that code is showing up because my cats have been falling apart.

Im saving up to get a tune hoping that it would fix the problem, I read that on some posts here.

Ive took off the intakes and put them back on thinking maybe Ive had a leak somewhere, and checked my map sensors but they're all good in there.

Also Im not sure how to check if I have ripped/cracked the plenum, if I remove the engine cover screw in the middle it causes a vacuum, I'm not sure if thats how it usually is!

I need to install test pipes because my car sounds like a moving bell lol

 

Hey, I am currently dealing with this problem and i cannot get my car smogged (california) because I have a light misfire and its stuck in the PDTC. I noticed i punctured the vacuum screw in the middle with the screw. I had the stillen gen 3 intakes and then took them off and sold them because infiniti told me to put back the stock intakes and it would fix the problem. Did that and replaced all the spark plugs and coilpacks did nothing. I was just wondering did replacing them whole plenum fix the problem?

 

So I figured out just put the screw in lightly into the plenum and then he hissing noise was gone. But the misfire wasn’t from that, it turns out it was actually from my catalytic converters. They got ripped apart inside, and the car wasn’t breathing properly, I put sea foam in with the gas, and that just blew the cats out of my exhaust, and the problem got fixed. So that could be one of the reasons why you have the misfire, check it out