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-   -   Kia Seltos 1.5L Diesel | Dancing, fluctuating RPM needle at idle (https://www.team-bhp.com/forum/technical-stuff/259672-kia-seltos-1-5l-diesel-dancing-fluctuating-rpm-needle-idle.html)

Back in July 2022, I noticed the RPM to fluctuate ever so slightly at idle along with an unsettled engine note. This got my attention when, unlike always, I felt a mild jerk when the car was rolling at 1st gear in traffic with any A input. Showed it to Kia service and they couldn't find anything wrong. So they just flashed the ECM (or at least that's what they claimed). I kept observing for the next few days and never saw it recur.

See around the 7 second mark, taken in July
https://youtu.be/LGqEciS_U5k

At first, I brushed it off as a one off incident and it could have been due to bad fuel. I had refueled at Chamarajnagar BPCL pump and I wasn't sure of it's quality (although I had burnt more than half the tank already when this issue started to occur in my car). But it happened again, to a different a different Seltos this time and in Chennai.

Last week, a friend (and BHPian) reported this issue in his GTX D AT, 20k run. He gave his car for service but they didn't find any issue (or at least they reported so). Back from service, it hasn't recurred.

And today, after returning from office and parking the car, I realized the engine note wasn't smooth at idle (whatever is heard within the cabin). I observed some more and realized the miss-fire with dancing rpm at idle. So the issue is back, that too barely 300 kms after the car was serviced. I don't think it is the fuel as I refueled at another bunk this time where I have been visiting for the past 4 months. I need to visit the service center again.:Frustrati

AC was off. Increase the volume to hear the uneven engine note, taken today
https://youtu.be/y034mBO4MEk

Lets take the obvious thing out of the way first considering you`ve just had it serviced.

See if the diesel filter is threaded in properly, sometimes these come from the vendor with a faulty gasket or seals.

Today, I pinged my service advisor at Epitome Kia and sent him the video. His first reaction was that the clutch is shot which is causing this issue. I tell him, the car is in neutral and he had nothing more to say. I have asked him to consult their chief technician if he has anything to say.

The diesel filter was changed a year/15k kms ago. You mean the oil filter? How would a loose oil filter cause the engine RPM to fluctuate?


Per my experience, there's no starting trouble and the RPM remains stable on a cold start. Each time I noticed this erratic idle rpm, I had driven 3-4 kms and the engine had just reached operating temperature. I couldn't perceive any impact on performance especially in the city traffic.
The other car where we noticed this issue, is a torque convertor automatic.

Diesel filter or any of the lines connected to it from the lift pump , if there is a bit of air gap it can cause this. It once happened to one of my vehicle due to a faulty diesel filter.

If the technician is referring to clutch then it can be the DMF, I'm not sure what kind of flywheel Seltos uses.

Diesel filter or any other portion fuel lines shouldn't have been touched during the recent service.

It uses a DMF. He could be right but I have my doubts. The idle has been smooth during cold starts and becomes unsettled only after driving around some time. Wouldn't a worn out DMF manifest itself at all times, including a cold start? Happy to be corrected though.

A failed DMF will cause the engine to hunt in the manner your car is doing. This happens only when the DMF heats up. That is why there is no problem during cold starts. Have it checked at the earliest before it manifests into other problems.

That it didn't happen for sometime in between could well be some mechanical empathy playing up - strange are the ways of the mechanical world. But check your DMF before its too late.

I've seen this even in my old DZire CRDI from day one. It's an extremely small variation (within 1mm on the RPM meter) and absolutely no perceptible/noticeable change of noise/feel in the real world. The car went on to cross 50k Kms with zero issues.

I think this can be considered a "rounding off error" for lack of a better term. It may neither be possible, nor worth bothering, to inject the exact amount or ratio of fuel/air mixture (in micro level measurements or variations) at every intake/exhaust cycle in every cylinder and injector.

However, if you can really hear/feel an uneven idling (make sure it's not the demons in your head) then it's probably something to check.

The reason I want to rule out DMF is that there are no other symptoms of DMF failing, such as engine vibrations, hard clutch or hard gear shifts. The issue doesn't manifest itself at all times, such as driving 200 kms on the highway or even 25 kms in heavy traffic. And all was well for 6k kms until this issue recurred.

The other car where we saw this issue, is a Seltos diesel automatic which doesn't have a flywheel. I want to be very sure before I let the service center open the engine for flywheel inspection/replacement.

Unfortunately the idling is uneven (can be heard clearly in the 2nd video I shared in my first post).

AT cars also would have a flywheel in most cases. That is how power is transferred from engine to transmission.

As I see it, The engine has no warning light. This indicates that the ECU does not see it as a problem. Even if the RPM is varying, it must be within the margin of error and hence no errors on car. You are not feeling any problems while driving. This further indicates it isn't a problem or not so big of a problem that you should be worried. At most it could be bad fuel or at worst it could be water in diesel. In both cases, adding Liqui moly diesel purge to the tank will help as it improves the cetane number of the diesel.

I see this issue occur sometimes in my Diesel AT as well. I assumed it had something to do with the DPF being clogged up, and that the car was doing something to clear it.

Why I say this is because I notice that if I just shift to P and leave the car to do it's thing for a while, the fluctuating RPM settles down and returns to normal. The whole process takes around 10 - 12 minutes or so.

And no, there was no DPF warning light or check engine light on the dash. I feel the car start to jerk at 1st gear speeds and I know what's up. So when it happens, I park the car somewhere, shift to P and just chill for a bit until the RPM settles.

And off I go, with the engine back to being as smooth as butter.


Note: I don't think it's bad fuel, because I am very picky with the fuel pumps I go to and choose trustworthy ones only.

I had exactly the same problem in my petrol Grand i10.
Got the throttle body cleaned and the problem was resolved.

There surely is a problem.

I drive a variant of this engine, the 1.6 Crdi since almost 65,000 kms and brushed off your first video as being a non issue for I too have noticed such fluctuations in the needle from time to time with no issues to report but the second video with that sound surely does not sound normal at all, there is way too much fluctuation to be heard in that, never experienced that.
Keep us posted.

Does the engine hunt/misfire similar to what I shared earlier? Last time it happened for me, I had asked the service center about DPF but they mentioned it was not clogged (but didn't mention what was the soot% by then). And the user manual says DPF regen happens only about 2000 rpm.

I have always wanted a soot gauge for reasons like this. That the service centers are clueless about such observations, only makes the situation worse.

It could be an unhandled exception too.:)

The ECU could also be programmed to lean out the fuel supply under certain conditions leading to such fluctuations. As few others have shared, there are more users out there facing this. How, why, when - these are questions which the service center can answer.



Looking at my thread, another user reached out to me on Insta because they are facing the same issue on their Hyundai Grand i10 diesel AMT and have shared the note below.