So, I just finished swapping my Cattman headers and a test pipe with electric cutout on the 4.5 gen. When I had a Warpspeed y-pipe with cat on there, the car idled great at around 700-750rpms. Now, the idle is bouncing between 400-750rpms. Sometimes while sitting idle at a stop light I have to feather the gas pedal because the car feels like it's going to die.

Anyone else have this issue? Engine has PFTB and is still using the 4th gen IACV.

Any help or input would be appreciated.

Thanks.

 

do 99s have EGR? maybe an EGR gasket vac leak?