Repeated spark plug failure after 2-3mths leading to misfire

[JB SA]

Kia Cerato/Cede 1.6 2013
I have a problem with repeated failure on a spark plug which leads to a misfire under moderate load at around 1300 rpm. Engine idles fine and runs fine beyond 1300 but over time, 1000km plus, it worsens until car runs permanently rough and goes into limp mode. There are no error codes. The injectors have been cleaned. Spark plugs have been replaced twice after which car runs perfectly for up to 1000km then it starts again, very intermittent and slight at first becoming progressively worse over time. No clear visible fouling etc of plugs. Any suggestions or thoughts. How to diagnose which cylinder is the problem and what the root cause might be?

[JB SA]

Thanks for response. Two mechanics and one auto electrician have checked with code readers, no code errors present.

[JB SA]

Agree it seems odd, looks like Kia engine management system does not keep record of missfire?? Both when it went into limp mode last year and subsequent occasion when we did not allow it to get that bad auto-electrician assures us there were no error codes.

Coil on plug

[JB SA]

Thanks all. Yes, in Kia Cerato case looks like its a matter of reading fault while fault occurs as misfire not stored in computer which is a bit difficult as it only occurs when driving not at idle. Have had three separate mechanics/auto electricians use their diagnostic devices so I am sure one or more of them had good kit. Looks like my next step is probably to replace coils and spark plugs and cross fingers and hope that is the problem. Probably cheaper than taking it back to a workshop where they charged me good money to clean the fuel rail and injectors which was not the problem.

[JB SA]

Thanks all. Replacing one coil and moving it from cylinder to cylinder will not work because over time, the fault causes the plug on the affected cylinder (not sure which one) to start performing poorly (although this is not the root of the problem) so you first have to replace the plugs to diagonse which cylinder is misfiring but once you do that the car runs well for about 2 months. I have not inspected the plugs because in both instances they were replaced by a workshop so did not see them after replacement.

The fuel injection is not GDI, think it is an EFI system.

Still looks like my best bet is either to hook up a diagnostic system while the car is driving to identify which cylinder is misfiring or to replace all the coils and plugs and cross fingers that sorts it out.

[JB SA]

Thanks Bearman and Arnie. Def sounds like we have a consensus here. I will go ahead with this plan of action.