Could be your MAF sensor
When you took the old intake assembly off the car you eliminated the Helmholtz resonator chamber. This device is used to increase volumetric efficiency as well as to muffle the intake noise. In most cases there will be a reduction in horse power without it. Also, you didn't mention whether or not you kept the mass air flow sensor intact. Big problems without that sensor. For more information just google "automotive intake resonator".So I’m new to Nissan club but I have a 2012 Altima coupe 6MT and I ordered a aftermarket air intake but when I installed it the car would start up and rough idle and then cut back off. I unhooked the battery while installing trying to reset the ECU but nothing. I can put the factory air box on and car fires up fine. Please help. Thanks in advance!