[Lee H]

Hi guys,  I have a 2021 PD I30 N-Liine,    had it now for 6 months,   I now have a very random miss fire, it's now happened 3 times,   the misfire is really bad with two cylinders,  But if you turn off the engine and restart it, it's fine,  been back to Hyundai twice and they can't find a problem.  took it back second time while it was still misfiring,  workshop foreman plugged in scanner to find 3 error codes 302,  304 can't remember the third, so misfiring on cylinders 2 & 4.  booked it in again to be looked at, and once again they can't find the issue.  They now say the spark plug gap is a little big so they have to contact Hyundai to replace them.   just wondered if anyone else has had a similar issue.. 

[Muzz258]

There are 2 lines of diagnosis for misfires.
1) Electrical
2) Fuel mixture.

Electrical includes things like coils, plugs, harness and the ECS.  The start here is to check the gap of the plugs, then swap coils with cylinders that are not misfiring.  If the misfire follows the change, replace the coil or plug swapped.

Fuel mixture require you to have a scanner.  A generic OBD2 scanner is OK.  OBD Fusion on your smart device with a EML327 OBD2 dongle is cheap and works well.  Read more about OBD Fusion here: https://www.e46fanatics.com/threads/obd-fusion.1240733/#post-18380161

You will need to look at the fuel trims.  There will be a short term and a long term fuel trim.  They are in %. The total fuel trim being applied to your engine is their sum.  Normal range is +/-4%.  If they are > +/-8%, then you have problems to find.  Most fuel trim/mixture problems are vacuum leaks.  These produce lean (positive) fuel trims.  Have a look at your fuel trims at hot idle and steady highway cruise 100kmph (60mph) and see how they line up.

If they are overly lean (positive) read this thread:  https://www.e46fanatics.com/threads/diy-fuel-trims-lean-codes-misfires-and-how-to-diagnose-p0101-p0174-and-more.1284608/

A smoke test is a good way to find vacuum leaks.  Read this thread:  https://www.e46fanatics.com/threads/how-to-smoke-test.1261077/#post-18615547

My sensible car is an i30 wagon.  My fun cars are old BMW's, hence all the BMW thread links.  The concepts are all the same and apply to your car.

If you get really stuck and want to fix the car yourself, run the 3 standard logs as described in the OBD Fusion link and post links to the CSV files.  I'll have  a look and see what's happening.  I do this sort of diagnostics all the time in the BMW world. 

[Lee H]

Thanks Muzz...
I'm deffinately thinking electrical,  as once you turn off the engine and restart it the misfire is gone,   it's going back in on the 29th for another assessment. so  we'll see what happens then.  hasn't misfired in the last two weeks which is good..    just makes it even more difficult to find the fault

[Lorian]

On those P codes the number on the end  is the cylinder number - 302=2 304=4

Plugs/leads first place to start I think.

[Lee H]

So we got the misfire diagnosed,  seems all cylinders are down on compression, all around 140,  should be min of 156,  found scouring in cylinder bores,  so now waiting for a replacement short engine.   not good for a 9 month old car with 20k     

[CraigB]

So we got the misfire diagnosed,  seems all cylinders are down on compression, all around 140,  should be min of 156,  found scouring in cylinder bores,  so now waiting for a replacement short engine.   not good for a 9 month old car with 20k   

First time I've heard of a issue like that thankfully it's under warranty.

[rein]

This thread if a wealth of information. I've been having misfires (and initial loss of power) for a few months now on cold-start, replaced plugs and coils and did a carbon flush (so much white smoke - rip GDi engines) but issue is still apparent.

I'm bringing it to Hyundai next week or so, will update with my info for others that are interested.

2019 PD i30 N-Line