there seem to be tons of idle issues threads out there so it made it difficult to weed through them for the correct answer, but I have a 1jz/auto ecu in my 92' 5spd sc300, it has been and still runs smooth and fine, but recently when i start it (even if its on a warm afternoon but its the first time that day the engine is started) the RPM fluctuates at idle until the engine warms up.

the most recent mod i did to my car was replacing the cat with a cut-out, leaving the O2 sensor that goes into the cat just under the car not in the exhaust system.

thanks

 

Bump, anyone?

 

Just wondering what you meant by O2 sensor "not in the exhaust system"?

 

the o2 sensor that plugs into the cat, i replaced the cat and the just hooked it up underneath the car, its basically hanging under my car in the outside atmosphere

 

So you have a 1JZ with a single 02 sensor, right? ...which isn't being used?
This just doesn't sound right

 

The O2 sensor shouldnt be a problem. The reason why the idle is unstable is because your tune is a bit off. Whenever that happens, it means the car is running rich in closed loop.

I would try resetting your ECU, or adjusting your tune with your AFC if you have one. But normally all heavily modified cars have this problem in closed loop, including mine.