The absence of fault codes proves nothing. The Engine Control Module is a computer, and just like any other computer it cannot see what it's doing, apply common sense or reason. It does EXACTLY what it's told. No more no less. A fault code will only be stored if it receives an input from a component outside of what it's been programmed to receive. You are human and have a complex brain. Just because you can experience a fault with your car does not mean the ECM also can.

Your description of the fault is fairly vague. If the fault is a misfire you may have an ignition coil tracking to earth. You said changing the spark plugs initially cured it, so this might be a place to start. The ECM will only set a fault code in this regard if a misfire is detected. That would come via the o2 sensor. The ECM does not control or monitor the high tension side of the ignition system.

If it improved a bit for while with new plugs then I'd suspect one or more of the coil packs. Usually get's worse in cold weather.

Worth having a test to see if valves are loosing compression with carbon.

Edited by Big John on 09/01/2019 at 21:40

A compression test is easy to do - and will prove if you have something wrong like carbon keeping valves slightly open - If left like this then it will cause valve burning.

Depending on inlet manifold you may be able to remove a sensor on it and insert an inspection camera to look inside the manifold and examine the back of the valves.

Coil packs easy and fairly cheap to change - especially if done at the same time as a compression test as you need to pull off coil packs and remove plugs to do this. I'd do this anyway!

Worth giving it a treat to to some higher Octane petrol such as Tesco Greenenergy so see if it helps a bit - Makes my 1.4tsi run better.

Edited by Big John on 10/01/2019 at 19:23

Quick update on this, over the weekend, I took each of the coils out one by one and checked all the spark plugs.

One of the plugs seemed like it was only just hand tight (I was cautious when I fitted them not to over tighten - seems l was too cautious). After putting it all back together again and tightening up the coils, it seems to be running much better. It's not super smooth, but that is probably just down to one of the other issues suggested above like the injectors being clogged, vacuum leak etc.

I'll see how I get on and run branded fuel for at least the next 3 tanks to see if that makes any difference.

Thanks all for the advice.