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Hyundai Elantra Revving at Idle

I have a 2002 Hyundai Elantra that is exhibiting some strange behavior at idle.
It wants to rev from 1500 to 3000 rpm, then back down, over and over again, every two seconds or so. This does not occur when the car is in drive or reverse; it purrs along at a consistent 1200 rpm.
It all started with overheating and a water pump change, which was completed and reassembled.
Then on startup, we began getting this revving. We double and triple checked the timing belt. We sprayed plenty of carb/choke cleaner in the areas of the vacuum hoses searching for a leak to no avail. Whenever we hookup the OBD code reader, there are no codes.
Why does my car want to challenge everyone to a drag race, and what is causing this?

If you’ve ruled out a vacuum leak, next I would check the idle air control valve (IAC). The IAC can be removed and cleaned, and sometimes that helps. Or it can be replaced if bad.

But I still wonder if the overheat didn’t warp something like an intake manifold and cause a vacuum leak between mating surfaces that you haven’t checked with your spray.

The problem might with Idle Air Control valve hunting for the proper position causing the idle speed to fluctuate.

The IAC valve controls the engines idle speed whenever the accelerator isn’t depressed.


I agree with both previous comments here. The IAC is more than likely at fault.

This does not occur when the car is in drive or reverse; it purrs along at a consistent 1200 rpm.

Could something be preventing the butterfly from closing at idle. 1200rpm idle in gear seems a bit high.

Concur w/above comments. I had a similar problem w/my Corolla last year and it turned out to be the IAC was faulty, plus the throttle body was a bit gummed up and had to be removed and cleaned. There are other possibilities, but that is a good place to start I think. Best of luck.

Thank you all. I’m hoping for the simple IAC swap.

IAC replacement did not fix the problem. Engine continues rhythmic surging from 1500 RPM to 3000 RPM in park and neutral. There’s no movement in the throttle. To confirm TDC position, timing belt was shifted one tooth over, which resulted in error P0340 (cam shaft position sensor error); surging continued despite timing belt shift. Timing belt shifted back to TDC, No codes at TDC confirmed. Butterfly has a good seal; still runs at 1200 RPM in gear. Any thoughts other than the aforementioned warped manifold? Thanks again for your help.

I think your intake manifold gasket is leaking air. See post #4 below. Owner had the same problem and it turned out to be a leaking intake manifold gasket.

fuel pump n fuel fillter drop metho down the tank as well as injector cleaner should get it

It is possible that after two years the problem has been solved or vehicle traded.

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