Rough Idle Issue with 2015 Cerato

silkysmooth on 28/04/2025 - 21:54
Last edited 29/04/2025 - 10:22 by 1 other user

Hi all, about six months ago, I purchased a 2015 Kia Cerato with a 1.6-litre engine.

Recently, I’ve noticed it has a rough idle (a slight shake) when stopped at traffic lights, in parking, and occasionally while reversing.

I’ve already replaced the engine mounts and spark plugs, but the issue persists. I also had the car inspected by a mechanic, but they were unable to determine the cause. The vehicle has recently had a full service as well.

Car has low km, 82000km.

If anyone has any advice or suggestions on what else I could check, it would be greatly appreciated.

Automotive Car Kia

Comments

MS Paint on 28/04/2025 - 21:54

+5

Ignition coil/s
- silkysmooth on 28/04/2025 - 22:18
  
  Thank you, my only symptom is the bad idle, can a bad coil have only this symptom ?
  - JIMB0 on 28/04/2025 - 22:30
    
    +2
    
    That's how it starts, then it eventually turns into a bad misfire and loss of power. The dead giveaway is that it's fine when cold and only happens once warmed up.
    - silkysmooth on 28/04/2025 - 22:32
      
      lol I was about to say that only happens when engine is warmed up, on start up everything is fine. Do I replace all four?
      - JIMB0 on 28/04/2025 - 22:35
        
        @silkysmooth: Scan for codes. Once the misfire is bad enough it should set a code telling you which one. If not you'll have to buy a spare and try swapping out each one until you find the bad one.
        
        It could be something else entirely. Find a mechanic that knows these cars and can diagnose it rather than one that just throws parts at it.
        
        silkysmooth on 29/04/2025 - 04:58
        
        @JIMB0: Thank you.
        
        I might go past supercheap and see if I can scan the car, does it have to get bad for the codes to go off?
        
        If I buy one, do I replace one at a time and then start the car and look for improved idle? What happens if there are two faulty?
        
        My wife did mention she could smell petrol when starting the car as well.
        
        I took it to two mechanics one through parts at it and the other gave up. Might look around and see another who might know these cars more.
        
        JIMB0 on 29/04/2025 - 05:52
        
        @silkysmooth: It's unlikely 2 are bad. A family member had a corolla that didn't set a code until it got bad.
        
        oscargamer on 29/04/2025 - 10:22
        
        @JIMB0: Yep, but if one has gone/is going, then the other 3 are likely to follow soon after
        
        JIMB0 on 29/04/2025 - 10:34
        
        +1
        
        @oscargamer: Not necessarily, at such low km it's just a random failure. The remaining 3 OEM coils will likely outlast the replacement aftermarket coil. At least on Toyota's this is the case. Genuine coils are about 3 times price of aftermarket.
        
        oscargamer on 29/04/2025 - 10:37
        
        @JIMB0: Sure, but is the OP wanting to chase the single? pack that is failing now by trial and error?
        
        JIMB0 on 29/04/2025 - 10:53
        
        @oscargamer: That's upto them, changing them is easy. I'm not familiar enough with Kia/Hyundai to know if the failure rate of coils is high and therefore if it's worth doing all 4.
        
        Like I said earlier it could be something else entirely. A proper scan tool and a mechanic who knows how to diagnose such things could be money well spent vs changing parts on a hunch.
        
        silkysmooth on 29/04/2025 - 17:32
        
        @JIMB0: Thank you,
        
        I might look for another mechanic.
        
        If I wanted to test the coil my self, could I buy just one and replace one at a time? And see results
        
        JIMB0 on 29/04/2025 - 18:01
        
        @silkysmooth: Being a tightwad I'd buy one and try it in each of the 4 positions.
        
        A decent scan tool should show the misfire count for each cylinder, it can cause a rough idle yet not meet the threshold to set a code.
        
        Also do some googling on this engine for common issues that might cause it.
        
        oscargamer on 29/04/2025 - 18:02
        
        -1
        
        @JIMB0: Yet you stated "It's unlikely that 2 are bad".
        
        JIMB0 on 29/04/2025 - 18:59
        
        @oscargamer: I did, what's your point? Disagreeing just to disagree?
        
        oscargamer on 29/04/2025 - 19:08
        
        -1
        
        @JIMB0: No. You state it's unlikely that 2 are bad but also state "I'm not familiar enough with Kia/Hyundai to know if the failure rate of coils is high and therefore if it's worth doing all 4" which makes me question your knowledge on this subject…..
        
        JIMB0 on 29/04/2025 - 20:42
        
        @oscargamer: Ah, I see how you're confused. Generally it will be just one coil failed. You can replace it with a great degree of confidence that the others will be fine. Some makes and models are known to have issues with coil packs, on these you're advised to change them all if one goes out, particularly if they're hard to get at ie. fwd V6. I'm sure op can use google to figure it out for this model.
        
        silkysmooth on 09/05/2025 - 18:00
        
        @JIMB0: Booked a mechanic next week to have a look, does it make a difference that it’s more noticeable in park and neutral? As soon as I put it in reverse it’s much less noticeable.
        
        JIMB0 on 09/05/2025 - 18:23
        
        @silkysmooth: There's more load on an engine when in gear so that would make it misfire more. Sounds like it could be something else causing it.
        
        I'm no expert.
        @oscargamer Thoughts mate?
        
        silkysmooth on 10/05/2025 - 08:32
        
        @JIMB0: Thanks mate appreciate the hints, I’m pickup up a coil this after noon and getting it scanned today. If I can work it out saves me taken it to mechanic next week.
        
        Just started it on cold start and no issue, I do smell a little petrol on the start to.
- darshu on 29/04/2025 - 18:57
  
  +1
  
  My Corolla behaved like this. The rpm suddenly drops when stopped at signal lights. I was worried the car might stall. I scanned the car, no codes. Changed fuel filter. Cleaned throttle body, mass air flow sensor. Changed spark plugs. Nothing worked.
  
  During last winter (after one year or 1.5 years since the issue started) when I stopped at the signal light, an ignition coil completely died and I changed all four coils. Now the car does not have the issue anymore. Apparently in Toyotas the misfire counts should be higher than a threshold to active check engine light or throw a code.
  - JIMB0 on 29/04/2025 - 19:02
    
    How many km and did you replace with genuine or aftermarket? I hope you kept the 3 good coils.
    - darshu on 29/04/2025 - 19:38
      
      I think my car had done 86k when the ignition coil died. I went to supercheapauto and brought an aftermarket coil to get the car running again. Amazon had oem coils each round $90 from Amazon Japan. They took around two weeks to arrive from Japan. Before ordering coils from Amazon, I rang Sydney Toyota. They told me each coil is $220 or $210 ( can’t recall exactly). I kept the good ones but not sure whether they are close to fail.
      - JIMB0 on 29/04/2025 - 20:37
        
        @darshu: Good info, thanks. We have an old corolla here with over 300k on it. A coil went out, Amaya price for genuine was $220, I'd hate to think what a local dealer would be. We just put an NGK from Repco in for $90. Wasn't going to spend too much into a 20 year old car.
        
        You could sell your old coils as spares, there seems to be plenty of demand, the wreckers charge $70 each.
        
        darshu on 30/04/2025 - 08:36
        
        @JIMB0: I had no idea the used coils cost that much :)
        Mine is a 2007 Corolla. It has only driven 90k so far. It's done a lot of short trips, probably that's why the coil died early (a lot of thermal cycles).
        
        silkysmooth on 30/04/2025 - 18:53
        
        @darshu: They very pricey, $340 for a pack of four.
        
        darshu on 01/05/2025 - 09:57
        
        @silkysmooth: Yes, it is. We only had one car, so I wanted the reliability. I always use OEM parts if I can afford it :)
SnowDragon on 28/04/2025 - 22:03

Could be misfiring? Happened to my corolla recently. Violent shaking when low/idle. There went $600
- silkysmooth on 29/04/2025 - 04:59
  
  Thank you, did you feel the miss fire or just shaking? Did you take it to a mechanic?
  - SnowDragon on 29/04/2025 - 08:46
    
    Had never experienced anything like it before. Was driving home one day and felt the car start to shake more than usual and especially more when idle or low revs.
    
    Took it into a mechanic the next day (worried the engine would die while stuck in traffic) and got a call an hour later saying it was a bad misfire.
    - silkysmooth on 29/04/2025 - 19:10
      
      Thank you, what car did you have ?
      
      Driving was it fine?
holdenmg on 28/04/2025 - 22:18

Fuel filter/ fuel pump?
- silkysmooth on 29/04/2025 - 05:00
  
  Haven’t gone down this path as both mechanics didn’t mention this but thank you
pegaxs on 28/04/2025 - 22:18

+2

Idle bypass valve? Air intake tube split, cut, damaged, leaking. MAP/MAF dirty.

I don’t know enough about this model to know what it has or not, but apart from bad coils/leads mentioned above, air leak could be a contributor.

Things like crank or cam sensor would more likely throw a MIL fault. Cam timing/jumped/worn belt would likely give bad running at all RPM.
- silkysmooth on 29/04/2025 - 05:07
  
  Thank you, a few things to go with. Might start with a scan. The mechanics I went to either gave up and said it’s normal or others just replaced parts and said just depends how much money i want to throw at the car. Hence I’m trying to trouble shoot my self.
  
  I might start with coil, then work through one by one.
AustriaBargain on 29/04/2025 - 07:30

Here are a few possibilities you could check next, based on that description:
• Vacuum leak: Even a small vacuum leak (like from a cracked hose or gasket) can cause rough idling without throwing a clear error code.
• Dirty or failing idle air control valve (IAC): If the IAC valve is dirty or sticking, it can cause unstable idling, especially at low speeds or when stopped.
• Throttle body needs cleaning: A buildup of carbon around the throttle body can cause rough idle because airflow is restricted or uneven at low throttle.
• Faulty ignition coil: Even if the spark plugs are new, an ignition coil starting to fail can cause misfires mainly at idle when the engine load is low.
• Fuel injector issue: A partially clogged injector can cause one cylinder to misfire slightly, noticeable at idle.
• Engine computer needing a relearn: After changing mounts or other work, sometimes the engine computer needs to “relearn” the idle through a procedure (especially on modern cars).

Since the engine mounts and spark plugs are new, vacuum leaks, IAC valve, throttle body cleaning, or a weak ignition coil would be the next most common suspects.
- silkysmooth on 29/04/2025 - 17:30
  
  Thank you
BOGOF on 29/04/2025 - 08:18

Check the PCV valve for any leaks and replace if split.
geekcohen on 29/04/2025 - 09:14

+1

I also had the car inspected by a mechanic, but they were unable to determine the cause.

Get a second opinon.
pao2x on 29/04/2025 - 09:59

Had the same problem with a yaris before. OBD error for Oxygen sensor in the ends

Rough Idle Issue with 2015 Cerato

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