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?

[E-bmw]

Get a code reader it will almost certainly tell you which is misfiring.

Check with the manufacturer that you are using the right spark plugs.

[JB SA]

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

[Krikkit]

If it's going into limp mode how can there be no codes?

Is it coil on plug or coil pack design? That would be my next port of call, assuming the spark plugs are the right brand and part.

[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

[NMNeil]

Fill a spray bottle with tap water, must be tap water for it's conductivity.
At night, or in a dark garage, with the engine running spray the finest mist possible over the engine.
Any high voltage path to ground will show up easily.

[E-bmw]

NMNeil said:

Fill a spray bottle with tap water, must be tap water for it's conductivity.
At night, or in a dark garage, with the engine running spray the finest mist possible over the engine.
Any high voltage path to ground will show up easily.

Have done exactly this myself with "old school" ignition systems alas as most are now ECU controlled individual coils it will show you nothing.

[The Road Crew]

JB SA said:

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

Perhaps the kit they have isn't good enough? What kit are they using?

It's very unusual that a car that drops into limp mode won't store a fault code.

[shakindog]

The Road Crew said:

JB SA said:

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

Perhaps the kit they have isn't good enough? What kit are they using?

It's very unusual that a car that drops into limp mode won't store a fault code.

As said above it’s a computer so should store historic codes even if the eml light has gone off.
Not all do so may be a case of getting it read when the fault is present. As anything else is just complete guess work.
If it’s gone thru a few plugs it’s usually a good idea to change coils also.
if you want to get trigger happy with the parts cannon.
Nothing worse than chasing an intermittent fault.
Kia did have a dodgy batch of injectors but far as I’m aware that was on newer cars and that caused a misfire.

Good luck with the chase

[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.

[georgeyboy12345]

Would it be worth buying a new coil and then trying it on each cylinder?

Or even just replace all four?

[Smiljan]

Coil packs would be a good next step, perhaps the new plugs are just giving a weak pack an easy time for a while until eventually the already weak spark on that cylinder becomes so weak it starts to misfire again.

I'm not familiar with this engine but if they're individual coil packs you could (as Georgey mentioned) just buy one and try it in each position to see if there's any improvement.

Have you also inspected the old plugs fully to see if there are any cracks in the porcelain or burn marks?

Hope that suggestion helps, it sounds like you really need the diagnostic on it while it's actually playing up.

[oakdale]

This model isn't available in the u/k so don't know the specifications, does it have the gdi engine?

[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.

[rustednut]

JB SA said:

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.

That does not make sense to me.

If you remove the plugs when it starts playing up, is it obvious which plug/cylinder is the is the issue or is it all cylinders/plugs?

If good diagnostic equipment cannot pick up a fault then start looking deeper, compression test being a good 1st step.

[bearman68]

I do this for a living, and this is what I would do....


Take it for a test drive, get the engine hot. Drive it up a hill at very low revs, really allow the engine to work at maximum torque at very low RPM.
If it's an ignition fault, it will misfire.
If there are no fault codes, it's because the misfire count is not high enough to trigger a EML.

But if it is misfiring, I'd change all the plugs and coils.

It's going to cost you maybe £150 to do that, but the ignition system is by far and away the most likely culprit, and I suspect the problem is the gap opening up in the plug causing strain in the coil.
It's possible to test for this, but the cost is going to e similar to the replacement parts - more effective to change the parts, especially as if one coil is gone, the others are close to failure.

If it doesn't misfire at low revs / high load, then it's unlikely to be the sparky system. (compression / oil rings, injector etc)

[NMNeil]

E-bmw said:

Have done exactly this myself with "old school" ignition systems alas as most are now ECU controlled individual coils it will show you nothing.

Not so. My old Ford Taurus had coil on plug and one of the coils had an almost invisible crack in the epoxy they use to seal it. Gave me a lovely firework show.

[Arnie Cunningham]

This. Your symptoms are exactly the same as my old Saab. New plugs fixed it for a month. New plugs AND new coil pack fixed it permanently.

bearman68 said:

I do this for a living, and this is what I would do....


Take it for a test drive, get the engine hot. Drive it up a hill at very low revs, really allow the engine to work at maximum torque at very low RPM.
If it's an ignition fault, it will misfire.
If there are no fault codes, it's because the misfire count is not high enough to trigger a EML.

But if it is misfiring, I'd change all the plugs and coils.

It's going to cost you maybe £150 to do that, but the ignition system is by far and away the most likely culprit, and I suspect the problem is the gap opening up in the plug causing strain in the coil.
It's possible to test for this, but the cost is going to e similar to the replacement parts - more effective to change the parts, especially as if one coil is gone, the others are close to failure.

If it doesn't misfire at low revs / high load, then it's unlikely to be the sparky system. (compression / oil rings, injector etc)

[E-bmw]

NMNeil said:

Not so. My old Ford Taurus had coil on plug and one of the coils had an almost invisible crack in the epoxy they use to seal it. Gave me a lovely firework show.

You are of course correct, but most coil on plug type has the HT end of things well insulated so it won't work.

[JB SA]

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