Misfires at idle

[JGR]

Hi,

Since I bought my 2017 Golf 7 (1.4 TSI 125HP, CZCA engine) two years ago, I feel rough idle.
Few times after buying it, I had a CEL so I brought it back to the dealer who told me that it was sporadic misfire on one cylinder.
They cleared the error, unmounted/remounted the spark plug and told me everything is fine, if there is no CEL, no problem...
So I drove with this issue for two years now but I was still worried about that and started to investigate myself on this.

What I found first, is that I have misfires counters which increment when I feel little shake of the engine.
I counted 15 misfires in about 2 minutes during my test at cold start.
Here are the actual values of misfires counters:
- Misfires cylinder 1: 1252
- Misfires cylinder 3: 1118
- Misfires cylinder 4: 1650
- Misfires cylinder 2: 941

I checked 2 spark plugs, because they had been changed by the dealer just before I bought the car (I was thinking they did not put the right reference...).
The reference is Ok and they seems well I think (pictures joined).

I launched a log at cold start to push further the investigation (file joined).
I am not an expert so it is hard for me to diagnose.
What I see is that :
- the STFT start decreasing when I feel engine shake (here it decrease until -7%)
- the LTFT decrease until -4,7% (when reaching this value and STFT near 0%, the engine is more smooth)

So it seems the mixture is too rich, but I do not know why and I do not know if it is the real cause of misfires.
Note that, I filled my last 2 tanks with SP98, before I used SP95E10.

I was thinking about an oxygen sensor issue, but it does not seems to be.
I saw on forums that it can be carbon build up in the intake that could be the issue but the car has only 70.000km...

Thank you for your help !!

Note that idle is rough at cold start but not only.
Also, no modification on the engine, only OEM pieces, always revised by the dealer.

 

[berserker6]

Unless I'm misunderstanding 1,650 misfires is insane in a day. Because the misfires are somewhat consistent across all cylinders I don't think it's a cylinder issue per se. (It's unlikely all 4 cylinders are shot) What tool are you using to diagnose and check your misfires?

 

[JGR]

Hi,
Thank you for your message.
Sorry I mispoke, they are not the values per day, they are the actual total counters (I think from the build of the car).
I retrieved these values from the Measured Values of the engine controler. No matter what the tool is, they are the same.

 

[JGR]

Hello,
I come back to my issue with new informations:

1. I did some logs at standstill but pushing the throttle pedal to increase RPM and it appears that misfires are also here ! Counters increase really quickly in that case but I do not feel it, no shaking, nothing... I have to do some logs while driving to see if it is the same.

2. I disassembled some motor part in order to insert a boroscope to view the intake. When disasembling, first I had little amount of oil in the intake pipe coming from the turbo to the intake manifold... Throtle is clear but it seems I have oil in the back of the manifold and the MAP sensor on top of the manifold had oil on it too...
I took some pictures of the intake valves. It appears there is some carbon build up on it but I have some doubts that this can cause the misfires.

What do you think ?

Thanks

 

[JGR]

Hi,
I come back with my issue which is still not resolved.
I did carbon build up cleaning of the intake valves but it changed nothing.

As I am pretty sure it is a air intake issue, I did some testing with the PCV pipes.

Three findings:
- Pluging off the pipe attached from the oil separator to the intake impacted the idle. RPM was not stable. And from monitoring point of view the STFT raised up and the Air leakage value also. Normally my Air leakage value is around 4,5 kg/h and I am not sure that this is the proper value. If someone can make a log of his own value (Vcds IDE01911) to compare I will be thankful.
- The throttle valve is totally closed at idle (I am not sure this is also the proper behavior). When choking the hole of the pipe described above in the intake, the car stalls.
- After putting everything back, I tried to choke the crankcase breather on top of the engine (connected to the air box), and nothing happened. The engine still running the same while it should stalls because it is normally the only air inlet.

So to me it looks like there is an air leak somewhere in the crankcase that allow too much air to enter in the intake at idle.
I am interested if someone has already met such air leak.

Thanks.

 

[swimming gti]

In a similar situation with misfires at 70k miles...sorry I can help too much as I'm not mechanically minded but interesting that carbon cleaning didn't fix it...the guy who looked at my car said he was 99% sure it was bc it had never been carbon cleaned

 

[EpicTech]

Replace the coil packs.

 

[JGR]

Hi,
Thanks for your replies.
@swimming gti In mine, the carbon build up was very small at 70k km, it depends on the engine, you can use a boroscope to check the intake valves as I did. But to cause issue, the carbon build up has to be much more worst than mine (see pictures above).

@EpicTech I think the issue does not come from coil pack as the issue only occurs at idle. There should also be some troubles at engine load if coil packs are failing.