Faulty spark plug/Limp Home Mode/EPC light - 2018 Polo Beats SE 1.0L

[hornedav]

Hello,

My 2018 Polo Beats SE 1.0L has done only 37,000 kms. From new it has had a range of technical faults, and has now been towed twice to VW dealerships for warranty repairs.

Latest issue involved faulty spark plug and carbon build up in cylinder 1. One evening car lost engine power, chugged, and went into Limp Home Mode. Barely made it home and to my driveway. Vehicle was again towed to dealership for warranty repair.

Fault was with factory installed spark plugs. Only 1 of the 3 spark plugs were replaced. I'm informed the new spark plug has a different Part Number to what was installed at factory. It is a mystery to me why VW would only replace one of the spark plugs with the newer model as I would assume the others will soon fail also.

Are 2018 Polo Beats known for faulty spark plugs?

Are 2018 Polo Beats known excessive carbon build up in engines? Can carbon be cleared from engine without disassembly?

Car is run on 98 RON fuel.

Thanks in advance,
David
Australia

[Andy Beats]

That's car manufacturers for you, they'll only replace what actually breaks.
Proactive doesn't exist if they're paying (yet they'll expect you to proactively replace perfectly good pads/discs early....)

So I'm guessing maybe the spark plug failed due to carbon build up?
Which suggests another fault, like a faulty injector or something that is making that cylinder run too rich.
And, no, carbon build up can't be removed without an engine rebuild.
It'd be like trying to remove plaque from your teeth without opening your mouth.

[monkeyhanger]

Using RON98 fuel may be a contributory factor here, if not the main factor. Why use RON98 on a RON95 optimised car? Do you think you're going to get X% more power or do you just like wasting money?

RON98 isn't more calorific than RON95, it is less volatile (via low level additives or substitution with Ethanol - which makes it less calorific) so can be compressed further without pinking, or you can put more fuel in per compression cycle without pinking - the main reason for using RON98, powerful cars need more fuel to achieve their high outputs, and when they don't (low output requirements), they run at a higher compression ratio.

Filling up with a less volatile fuel is far more likely to result in an incomplete burn, with resultant carbon build up. Of course if that's the case, the other 2 cylinders will be coking up too and the other 2 spark plugs won't be far behind.

Another cause could be a dodgy coil pack for that one spark plug.

Just to demonstrate calorific value vs RON, pure ethanol has a RON number of 108,but only 85% of the calorific value of petrol. As a result, ethanol optimised cars are both powerful and thirsty (needing 18% more fuel to perform the same work, but can put in about 40% more fuel under a higher compression ratio without pinking.

[Andy Beats]

Using RON98 fuel may be a contributory factor here, if not the main factor. Why use RON98 on a RON95 optimised car? Do you think you're going to get X% more power or do you just like wasting money?

RON98 isn't more calorific than RON95, it is less volatile (via low level additives or substitution with Ethanol - which makes it less calorific) so can be compressed further without pinking, or you can put more fuel in per compression cycle without pinking - the main reason for using RON98, powerful cars need more fuel to achieve their high outputs, and when they don't (low output requirements), they run at a higher compression ratio.

Filling up with a less volatile fuel is far more likely to result in an incomplete burn, with resultant carbon build up. Of course if that's the case, the other 2 cylinders will be coking up too and the other 2 spark plugs won't be far behind.

Another cause could be a dodgy coil pack for that one spark plug.

Just to demonstrate calorific value vs RON, pure ethanol has a RON number of 108,but only 85% of the calorific value of petrol. As a result, ethanol optimised cars are both powerful and thirsty (needing 18% more fuel to perform the same work, but can put in about 40% more fuel under a higher compression ratio without pinking.

First time I've ever seen anyone mention that using super unleaded in a car that doesn't need it actually results in more carbon.
Do you have any links to anything I can read that says this?
If anything the premium petrols contain additives that should reduce the normal carbon build up in an engine.

[Harrihealey02]

When RON 98 Is used in normal engines tuned for standard 95, you're not going to benefit your engine because the higher RON simply means it won't ignight unintentionally under extreme pressure, which is why most sports car require it due to the piston TDC having more compression than standard engines like yours. Most fuel stations use more cleaning additives in the higher performance fuels which will help a normal engine. The cost of the premium fuel will not make up for the extra cleaning additives though. Taking it on a long drive in low gears once a month will clear out the carbon etc.

[Andy Beats]

When RON 98 Is used in normal engines tuned for standard 95, you're not going to benefit your engine because the higher RON simply means it won't ignight unintentionally under extreme pressure, which is why most sports car require it due to the piston TDC having more compression than standard engines like yours. Most fuel stations use more cleaning additives in the higher performance fuels which will help a normal engine. The cost of the premium fuel will not make up for the extra cleaning additives though. Taking it on a long drive in low gears once a month will clear out the carbon etc.

Going for a drive in low gears once a month to avoid carbon deposits in your engine is extreme behaviour I've never heard recommended before.
And how do you work out what's better, using cheap petrol and going for a thirsty drive at high revs one a month, or using the dearer fuel that will reduce carbon deposits via the additives and forget the long runs?
Seems like a complicated calculation, that one.
All these allegedly hyper efficient cars that need unnecessary high-rev drives now.....