The exhaust can cause problems if you have leaks around the downpipe area. The reason this can cause a problem is because a leak near the downpipe will cause the 02 to be off from the air that will enter in the area of the leak. The 02 getting a false reading will absolutely cause idle issues. The other items that Dane pointed out are all items that can cause problems as well.
:tight throttle cable :throttle bodies not sutting all the way :throttle bodies not synconized :vacuum leak :timing :base TPS :ECU temp sensor
