1.2 HTP 2008 rough running - dodgy coil?

[Rita]

HI,  i've just bought a low 38k mileage Fabia,  in very good condition overall, however it has an intermittent fault.  it runs OK when cold, but after about 20 minutes of running, it starts to run roughly, feels like it is only firing on 2 out of 3 cylinders, and will generally pick up if the accelerator is floored.  Having looked on the forum, my  suspicions are a dodgy coil, however plugs look original ( branded VW), but good colour,  and had gaps of 1.5mm!  So, new plugs ordered as a first step, then on to coils.  My question is that at no time do i get any engine management warning light, which i was expecting if  a coil is dodgy?  Would this imply something else?

[xman]

Hardly surprising it runs rough if gaps worn to 1.5mm which is over twice the correct gap iirc. New sparkplugs please (about £9 for a set from ecp or gsf)

 

Coil packs will usually fail completely. Probably not running rough enough for long enough to trigger engine light, which will go out after the engine starts running smoothly on all 3 cylinders.

 

Why is the gap 1.5mm? Has the centre or ground electrode worn away that much? If so, from the experience of our two htp engined fabias, I would question if its really only done 38k.

Edited 15 January, 2018 by xman

[Rita]

Yes xman,  i agree, with gaps like this not surprising it was running rough, but having adjusted the gaps, it's still a bit rough. I'm collecting new plugs this evening. 

 

Engine light doesn't come on at all, which makes me think it isn't the coil, but i'm not sure what would trigger the engine warning light, engine vibration or an electrical fault sensed by the car electronics?

 

  No reason to suspect mileage is incorrect - engine looks like new, and from the DVLA MOT checker it has been only doing 3k miles a year since 2011, but i suspect on short journeys in heavy traffic in Bristol, so the mileage isn't a good reflection of the engine run hours. 

 

i'll fit the new plugs and see how it goes.

[Rooted]

^^^ 
so what plugs have you ordered,

and do you know what the gap is so you can check the pre-gapped plugs before fitting them?

[Rita]

i've ordered NGK 5960    ZFR6T-11G plugs as per the NGK website,  i believe  gap should be  0.8mm?  i'll check gap on supplied plugs before fitting.

[xman]

6 hours ago, Rita said:

No reason to suspect mileage is incorrect - engine looks like new, and from the DVLA MOT checker it has been only doing 3k miles a year since 2011, but i suspect on short journeys in heavy traffic in Bristol, so the mileage isn't a good reflection of the engine run hours. 

 

i'll fit the new plugs and see how it goes.

 

I hope they changed the oil every year! Htp engines with neglected oil changes are known to develop valve guide wear, with the consequence that valve seats go oval.

 

Keeping fingers crossed that spark plugs fix it for you.

[Rita]

Just to complete this thread,  having changed the plugs, air filter and oil, no improvement.  i then ran diagnostics which indicated a failed lambda  sensor, and i started a new thread  https://www.briskoda.net/forums/topic/443976-what-does-this-symbol-mean/  , so the saga continues......