High idle problem...please help!
 

Hey guys I have an S4 n/a and has recently devloped a high idle problem. The idle stays at about 3,000rpm even when warm. I checked the bac as that seems to be a common problem but seems fine. I have a 170 degree thermostat instead of the 180 it calls for i dont think that is the problem because the car heats up. Please help me out i have no idea what is going on.

im no rotary Genius but you stated that you have the 170 stat instead of the 180? have you tried changing it? or did you make sure that you stat is not installed backwards.. i have heard that heat ups happen cuz of that too. just in idea

Get an OEM 180 degree thermostat to start. If the thermostat is not opening, no hot coolant will reach the radiator, which will never trigger the switch at the bottom corner. It switches the cold start 3000 RPM idle off when it reaches ~60* F, so if it has no coolant flowing to it it'll never trigger. But the engine should still only rev up to 3000 RPM for about 17 seconds; it should never be permanent.

You can also try unplugging the BAC and AWS solenoid valve, which are the sources of airflow for this system. If the problem goes away, you know it's an issue with them or the control system (ECU, temperature switch). If not related to these, you've probably got a throttle stop that's been played with, the throttle cable is sticking, stuck thermowax, and/or the air bypass screw on the TB is open too far.

Don't forget to check for vacuum leaks, those will cause the engine to rev high at idle...

your throttle isnt stuck is it? take the intake tube out and see if it closes all the way.