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?
Check your coolant levels and maybe get a leak down test done. Air trapped in the coolant system could be messing with some type of fast idle valve. Pull codes and check from there also!
Edit: Coolant in some cars passes through a type of fast idle valve that raises and lowers a plunger(connected to some type of heat reactive wax that closes the airway fully when the coolant is at OT) through a tube that feeds more or less air through the engine raising or lowering its speed. If air is trapped in the system it can in some cases cause a fluctuating idle as that wax isn’t heated consistently. I don’t know however if this is completely your case, as this I’ve seen this used on cold starts to warmup the car faster which is about 1-5-2k rpm, not 4k rpm
