I have a 2001 Hyundai Elantra GT.
As of late when I start it up it idles rough for a few seconds, then revs up to 4000RPM and back down to 1000RPM. This continues until the ignition is switched off.
I have replaced the throttle position sensor, manifold pressure sensor and idle speed control in addition to the downstream O2 sensor (upstream O2 not replaced).
I'm at a loss, any ideas about what's going on?