ok well my friend just got his 1993 240sx ka24de coupe and all it has done is full injen cold air intake and full 3" exhaust it Idles at like 1500-1200rpm and some times when u push the clutch in and stop the car stalls and the engine light is off and does not over heat what could it be
First, make sure you don’t have a vacuum leak - so just ensure that all the intake’s connections are tight.
Then make sure your MAF is relatively clean - as in the wire isn’t coated in oil etc (be careful with it).
Then try cleaning out the throttle body and IACV valve.
The passage to the IACV is the hose that comes of your intake (near the throttle body) and clamps onto the metal tube heading towards the engine. Spray throttle body cleaner liberally into that pipe, let sit. Car may need a few starting attempts and some revving, then let the car run/rev for a while…alternatively you could have someone revving the car while you spray.
IACV problems are common and usually result in idle hunting when the valve sticks open/closed and/or random stalling when the valve doesn’t react properly.
It this helps notably but only short-term and a few cleanings don’t fix it, it may need replacing. If it doesn’t help at all, your problem is likely elsewhere…
Good luck.