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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?

  • There may be an idle relearn procedure. Is there any correlation with anything switching on and off? Vacuum leaks? What happens if you disconnect the IAC? – Ben Feb 7 '18 at 19:49
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    the 2nd O2 sensor usually has very little (usually nothing) to do with how the engine runs. the 1st one closest to the engine does pretty much all the work for air/fuel ratio in most non-new generation cars. – agent provocateur Feb 8 '18 at 0:56
  • Does this behaviour go away when the engine is fully warmed up? Or have you not left it running for that long? – Zaid Feb 9 '18 at 3:13
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    Leaving this as a comment, since it is just a shot in the dark. My 2001 Elantra did this exact thing once after a repair and it took me far too long to figure out. I had accidentally moved the cruise control cable and it was caught on something. With the extra tension, it was doing something to the module to rev the engine up to around 4000 RPM and back to idle-ish every few seconds. I'll convert this to an answer if it turns out to be the problem :) – Poisson Fish Feb 9 '18 at 20:57
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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

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