[Maciej]

Hi guys,

I have a 4 year old i30 which sometimes revs up & down on idle (fluctuates). I have researched this topic a little and it seems there might be few culprits like dirty or old O2 / mass airflow sensors, dirty throttle or even vacuum leak. I had the car inspected by my mechanic but he didn't discover anything suspicious (the fault is intermittent so probably didn't occur). It is very annoying as I feel the whole chassis trembling in the rhythm. I am now trying to narrow down the suspect list - maybe my mechanic will follow the lead.
I was observing the symptoms for a good while and I can say with 99% certainty that:

1. It starts only after the engine is really warm - at max of engine temp gauge. In rare situations where the engine is allowed to cool down so gauge drops one bar - the problem disappears.
2. It also temporarily disappears when I switch on AC and starts again the second I switch it off.
3. The moment fluctuations begin, the Start/stop system becomes inactive & stays like that until engine is allowed to cool down.

See below screenshots from Torque. You can clearly see that all graphs are usually flat & steady and peaks&dips start to show the moment fluctuations begin. Almost all sensors seem to be affected - LOAD, REVS, MAF, O2S1EQ and BOOST.

See revving here:

VID 20180410 081602 - YouTube

Anyone got any suggestions? Or this is normal and I'm just overreacting?

[nzenigma]

Mate thats a great post, unusual to get so much detail. Thanks.

Coincidentally, I have just replied to a guy in Lithuania who has 'similar' "tremble' in his CRDI.
 Your "old O2 / mass airflow sensors, dirty throttle [sensor] or even vacuum leak" were also my suggestions to him. I assume you have explored these?

Looking at the data, I have a gut feeling that the fault is not in any of those areas but some other sensor is amiss.

I say sensor, because it is heat sensitive and intermittent. I would change the crank position sensor. It has been a culprit in the past and is an easy fix.

@tw2005  What say you?

[Dazzler]

Mate thats a great post, unusual to get so much detail. Thanks.

Coincidentally, I have just replied to a guy in Lithuania who has 'similar' "tremble' in his CRDI.
 Your "old O2 / mass airflow sensors, dirty throttle [sensor] or even vacuum leak" were also my suggestions to him. I assume you have explored these?

Looking at the data, I have a gut feeling that the fault is not in any of those areas but some other sensor is amiss.

I say sensor, because it is heat sensitive and intermittent. I would change the crank position sensor. It has been a culprit in the past and is an easy fix.

@tw2005  What say you?

Thanks for thanking him Gary! 

I thought the same thing. You beat me to it!

[Bogdan1986]

Hey Maciej. Since the car is 4 years old, I assume it is still under warranty. Did you contact a Hyundai service department about your problem? Or maybe your mechanic works for Hyundai?
From experience, dealerships are most of the time useless when you have an intermittent problem, they don't spend enough time investigating so they can't fix it. Still, it might be worth it to ask for their opinion.
I have a similar problem with my i30 FD model, but only appears when the outside temperature is over 16C and it is also accompanied by acceleration lag and an EGR fault code. Right now, since it's very cold outside, the car runs perfectly.
Did your mechanic check the EGR valve? From what I understand, it is commanded shut when the AC is on. Monitoring it with the torque app wasn't much help for me, the app is too slow to pick up my problem. I purchased a Hyundai VCI clone and made a few screenshots, compared them to a screenshot from another forum member that fixed an identical fault in a cee'd and it was a match. My ECU is faulty, for some reason, it doesn't control the EGR valve as it should and it returns a false EGR trouble code.
Before I found this out, I changed the MAF, boost pressure sensor, EGR valve and camshaft sensor without any change. I suspected the O2 sensor (or air/fuel sensor), but it shouldn't give an intermittent problem if it was faulty.
It might be a good idea to check and clean the EGR valve.

[nzenigma]

Hi @Bogdan1986  , nice reply mate. Some useful info for my brain's file  , thankyou.

Your comments to Maciej could help him.

 I considered EGR but Maciej has an intermittent fault ( could be ECU)  but unlike him, your fault is permanent.
 I also thought EGR was off when A/C active, but that has been challenged in another post and I didnt have time to explore it further. Not Sure.
Have you tried to delete your EGR valve? ( blanking plate)

[Bogdan1986]

Hey nzenigma, I didn't try the blanking plate... I am sure that if I did install one, the check engine light would stay on all the time. I would have to remove it every time I went for an MOT too, they check the car thoroughly here. What I will do next spring is buy a used ECU, I can find one for 200-300 euro on ebay.
As for the AC shutting down the EGR valve completely, it is true it does not work like that for my car. Mine acts the same with AC on and off. However, I met with a fellow forum member that lives in Denmark and has the same problem, we tested his car with the VCI, too. We saw clearly that the valve shut closed and stayed closed whenever the AC was on. We think the workshop that worked on the car when he started having issues remapped the ECU to do that in an attempt to fix the issue.

On a side note, I have a friend that experienced intermittent idle rev fluctuations in a Opel Insignia and in his case, they stopped after he cleaned the EGR valve.

[nzenigma]

hi mate, sounds like Romania rocks in spring. I had a great time in Canada, parties everywhere as the place thawed.

You probably know this, the EGR plate has a 5mm hole in it. Enough gas passes through it to fool the sensor.

Thanks again, I found both your posts instructive. Cheers Gary.

[Arek]

HELLO
I had something similar but it wasnt going so rapidly. In my case it was this part
KIA RIO II III CRDI OPORNIK REZYSTOR WENTYLATORA

If its not working properly the fan from AC is runing up and down speed up and down. Its really easy to find it under the hood somwhere in the fun of ac left or right side.  This part should be  really hot if it not you found a problem. I bought used part and now its working really smoth.

[Dazzler]

  Arek and thanks for your input.

Interesting. That equates to only about A$35

[nzenigma]

Arek and thanks for your input.

Interesting. That equates to only about A$35

Thanks from me too 

[RayM]

Hi, this is a total shot in the dark because I have no mechanical knowledge of cars, but as your car is 4 years old, could it be something as simple as a failing battery? My old Getz from a few years ago was doing a lot of odd things like not pulling away from a stop etc. I paid a garage over £300 to fix it, they didn't, but when I later replaced the battery all the problems went away.
Good luck.

[sundiz]

How is your battery and alternator? It came to my mind that it only happens when engine is really hot. Could it happen when the electric fan goes on in the engine bay? My rpm does not fluctuate, but the idle is definately rougher when the cooling fan creates some load for the alternator.