Issues with my Kia Seltos: Fluctuating idle RPM & unusual engine note

Took the car to the service centre but they couldn't find anything wrong so they just flashed the ECM.

BHPian ashis89 recently shared this with other enthusiasts.

Back in July 2022, I noticed the RPM 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 in the above video that was taken in July.

At first, I brushed it off as a one-off incident and it could have been due to bad fuel. I had refueled at the Chamarajnagar BPCL pump and I wasn't sure of its 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 Seltos this time and in Chennai.

Last week, a friend (and BHPian) reported this issue in his GTX D AT, 20k km 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 the 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.

AC was off. Increase the volume to hear the uneven engine note. The video was taken today.

Here's what BHPian Speed.Demon had to say about the matter:

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 its 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.

Here's what BHPian Kosfactor had to say about the matter:

Let's 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.

Here's what BHPian EV NXT had to say about the matter:

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 some time in between could well be some mechanical empathy playing up - strange are the ways of the mechanical world. But check your DMF before it's too late.

Here's what BHPian PearlJam had to say about the matter:

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.

Check out BHPian comments for more insights and information.