What are a sing of a bad Idle control valve, will this cause bad mpg?
No, the valve is there to provide a steady idle regardless whether the transmission is in gear or the air conditioner kicks on. If the Idle Air Control (IAC) valve is not working right the engine will usually idle low and try to stall, although it can also cause high idle.
Generally, a bad ICV will either be stuck open from carbon or dirt making the idle stay too high. Or dead which usually kills the engine when put in gear. Mileage would be affected, but if it idles good, it's probably ok.
