Misfire under load in 5th/6th gear - plugs or coils or both?

[funkyg]

2020 Polo GTI. On a couple of occasions recently my car has misfired when accelerating hard on the motorway, in 5th or 6th gear. Gentler accelration in these gears doesn't misfire. If driving normally around town I also feel no misfire. Also no misfire while cruising steadily on the mototrway. No enigine management light showing.

Car currently has 26K miles. Am on original coil packs and plugs were last replaced two services ago around 10K miles ago. Next service is due in March and as I'm only doing around 5K miles a year was going to get the basic service again. But with this will probably get the plugs as well. Is it worth getting coils as well or just see how I get on with the plugs first?

[silver38]

I had this exact problem on two seperate occasions. Turned out to be coil packs and one associated plug with a strange colour.

[funkyg]

Was going to replace the coil packs in the new year but my car had other ideas.
Driving up the motorway on NYE and the car started misfiring badly and juddering with a loss of power. Pulled over and got towed home. Took it to the dealer while I still had warranty and they diagnosed carbon build up on intake valeve and plugs. They walnut blasted the valves and replace the plugs (neither covered by warranty) and also put a new set of coils on, seems fine now. Not sure if it's just in my head but the engine seems smoother than ever now anyway.

[SRGTD]

@funkyg; good that’s it’s been sorted and is now running as it should. 26k miles does seem quite a low mileage to need walnut blasting to remove carbon build up on the valves though (possibly a consequence of poor fuel quality?).

[OomStu_ZA]

@funkyg; good that’s it’s been sorted and is now running as it should. 26k miles does seem quite a low mileage to need walnut blasting to remove carbon build up on the valves though (possibly a consequence of poor fuel quality?).

Heya SRGTD, I thought carbon build up was not a thing in the UK due to better fuel than we have. I've had mine done twice now (roughly every 50 000KM) and expecting the next session soon since I'm now on 135K KMs.

[SRGTD]

@funkyg; good that’s it’s been sorted and is now running as it should. 26k miles does seem quite a low mileage to need walnut blasting to remove carbon build up on the valves though (possibly a consequence of poor fuel quality?).

Heya SRGTD, I thought carbon build up was not a thing in the UK due to better fuel than we have. I've had mine done twice now (roughly every 50 000KM) and expecting the next session soon since I'm now on 135K KMs.

I think carbon build is more likely to be an issue with direct injection engines. The engine in the pre-facelift Polo GTI (2.0 EA888 Gen3b) has port injectors to wash the backs of the valves with fuel vapour to help minimise carbon build up (don’t know if the facelift GTI retained port injection). With decent quality fuel, I wouldn’t have expected carbon build symptoms (e.g. uneven idling, hesitancy, misfiring) to start to become an issue until after circa 60k miles.

As for fuel quality - it’s suggested on some forums that in the UK, supermarket branded fuel may be a lower quality than some of the well known branded fuels such as Shell, BP and Esso. The base fuel is likely to be the same as it usually (always?) comes from the same refineries for all fuel retailers, but the additives to the base fuel will differ by brand - I don’t know but maybe the additives are more concentrated - and maybe more additives in the blend used for the well known brands compared to supermarket fuels?

[OomStu_ZA]

@funkyg; good that’s it’s been sorted and is now running as it should. 26k miles does seem quite a low mileage to need walnut blasting to remove carbon build up on the valves though (possibly a consequence of poor fuel quality?).

Heya SRGTD, I thought carbon build up was not a thing in the UK due to better fuel than we have. I've had mine done twice now (roughly every 50 000KM) and expecting the next session soon since I'm now on 135K KMs.

I think carbon build is more likely to be an issue with direct injection engines. The engine in the pre-facelift Polo GTI (2.0 EA888 Gen3b) has port injectors to wash the backs of the valves with fuel vapour to help minimise carbon build up (don’t know if the facelift GTI retained port injection). With decent quality fuel, I wouldn’t have expected carbon build symptoms (e.g. uneven idling, hesitancy, misfiring) to start to become an issue until after circa 60k miles.

As for fuel quality - it’s suggested on some forums that in the UK, supermarket branded fuel may be a lower quality than some of the well known branded fuels such as Shell, BP and Esso. The base fuel is likely to be the same as it usually (always?) comes from the same refineries for all fuel retailers, but the additives to the base fuel will differ by brand - I don’t know but maybe the additives are more concentrated - and maybe more additives in the blend used for the well known brands compared to supermarket fuels?

Ah ok, thank you for the explanation. In SA we have Shell, BP, Total (pronounced Totaal = French I think), and some "new" brand called Astra I think. We also had Sasol which was locally produced but since I'm in the Western Cape we dont really see their service stations here, mostly up country. Now Shell is about to pull out of SA (politics) so the new "owners" will change it to something else.

[funkyg]

@funkyg; good that’s it’s been sorted and is now running as it should. 26k miles does seem quite a low mileage to need walnut blasting to remove carbon build up on the valves though (possibly a consequence of poor fuel quality?).

I hold my hand up to poor/cheap fuel. The only fuel station near me is Asda so that’s pretty much all I’ve used. I’m going to make an effort to use better fuel like Shell/Esso. Is it worth going full V-Power or just regular but branded fuel?

[RUM4MO]

I'd tend to use only what you hope is a quality branded fuel and just use the "U/L" version normally and a complete tank of "U/L +" once or twice a year so that means benefiting from the "quality" additives package when using the basic fuel and the same when using the premium fuel using its higher loading of "quality" additives to clean the engine up a bit - ideally try to tie in your once or twice a year use of premium quality branded fuel with a long journey if possible to hopefully make best use of the cleaning up effect.

[SRGTD]

@funkyg; good that’s it’s been sorted and is now running as it should. 26k miles does seem quite a low mileage to need walnut blasting to remove carbon build up on the valves though (possibly a consequence of poor fuel quality?).

I hold my hand up to poor/cheap fuel. The only fuel station near me is Asda so that’s pretty much all I’ve used. I’m going to make an effort to use better fuel like Shell/Esso. Is it worth going full V-Power or just regular but branded fuel?

I read somewhere a good while ago (sorry, can’t remember where) of people being critical of Asda and Morrisons supermarket fuel and it being at the lower end of the quality scale - whether those views were based on some form of objective research or just baseless opinions I have no idea. I’ll also hold my hand up to using supermarket fuel - Tesco - as the local Tesco filling station is near and handy to me and I object to driving miles out of my way to buy fuel. I’ve used it pretty much exclusively (unless I’m on a long journey away from home) for over 20 years; diesel, unleaded and Momentum 99 premium / super unleaded and never had issues with any.

On the subject of regular v’s premium fuel; I initially used premium (Tesco Momentum 99) in my 2016 6c Polo GTI until at one point a few years back there was a fairly significant price increase. At that point I switched to normal unleaded and in all honestly I noticed no discernible difference in performance or fuel consumption between the two, so stuck with normal unleaded from then on. My 2020 Polo GTI+ was just run on normal Tesco unleaded during the 4 years I owned it. However, if I’d had issues with carbon build up I would have considered switching to the premium, higher octane Momentum 99 or at least filled up periodically with the ‘good stuff’ to get the benefit of the supposedly superior cleaning properties of the additives pack in Momentum 99 compared to normal unleaded.