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About a month ago my 2003 Hyundai Elantra's check engine light came on. Afterward I noticed that a couple minutes into my drive it would slow and although I pressed on the accelerator it seemed to barely react. Over the weeks the problem has continued to get worse. Now when I start my car I wait for about 3 minutes for it to do this same behavior. The RPMs drop down and it acts as though it may stall and it runs very rough. I leave it in park the whole time. After about 30 seconds it seems to correct itself and suddenly the RPMs rise as though I'm flooring it for a couple seconds before dropping. After this my car runs fine for my 45 minute commute both in-town and on highway driving. I haven't been able to find anything else on forums that sound like it.

  • Did you have the check engine light (CEL) read/scanned? Most auto part stores will do so for free. – MooseLucifer Jul 21 '16 at 22:31
  • No. I have been in the middle of a crazy project at work, coupled with the commute it doesn't leave me much time. Just wanted to get an idea as it seems fairly specific. – Zombian Jul 21 '16 at 22:36
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    It sounds like a faulty sensor. It could be coolant related since it happens after you let the car warm up for a few minutes, could be a mass air flow or o2 sensor. You should go have the code checked, the parts store guys are paid by the hour so they won't try to sell you stuff. They're there to help, and if it's a simple problem they'll be able to advise you on a fix, it takes all of 5 minutes. At the very least it will help us figure out what's happening. – MooseLucifer Jul 21 '16 at 23:01
  • I think it might be your TPS or fuel pump. They are notorious for both. Post the CEL code. – cloudnyn3 Jul 22 '16 at 1:37
  • I also agree with the faulty MAP/MAF or O2 sensor theory. Get your engine code checked, it will likely tell you exactly what the issue is (you can generally get the code read at e.g. Pep Boys, places like that, you can also buy OBD/bluetooth interfaces online for like $15 and check with a smart phone). If it's the air pressure/flow sensor swap it out and swap out your air filter too if its dirty. The air sensors (and filter) are generally a super easy diy fix, the O2 sensors slightly more challenging but still not bad, and all those parts are often available at the local parts shop. – Jason C Jul 22 '16 at 3:45
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I would test the IAC (idle air control) system. There are two valves, a mechanical one that operates in open and closed loop mode (open : engine not warmed up, closed : engine warmed up), and a electronic controlled valve that only operates in closed loop. If the issue only happens once warmed up and driving around the IAC is most likely your problem, although i would test it.

Faulty throttle position sensors usually cause loss of power followed by surges. Faulty IAC usually cause surges followed by loss of power. Know that difference can help diagnose.

The reason for this is when the IAC valve fails the engine vacuum forces the valve open hence the engine surges, then the ECU trys to compensate to bring the engine back down. When a MAP/MAF or throttle position sensor fail, the ECU just gets erroneous data that your foot is no longer on the pedal or there isn't enough air so less fuel goes into the combustion chamber... so you lose power. Then once the sensor is past the blind spot the ECU realizes the error and surges do to overcompensation.

So does the engine lose power then surge or vice versa? If both you might have a handful of issues that need diagnosing.

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