Hi Scott,
Ok so here goes…. I’m actually having a go this weekend but here is what ive found so far.
Basically this is 1 or all of 5 things, no more, no less! Ive used a VAG come unit (Laptop and cable) to get the fault codes and lots or internet research to avoid trips to garages although ive been to a few who said (as air whistled through their teeth) No mate could be anything that!!
1. Faulty MAF - This has been changed on mine, best way to see if its this is run it with it disconnected i.e. disconnect the electrical connector but leave the unit in place. Run the car as normal! If there is no or little difference its unlikely to be your MAF. This is the unit that runs between your airlifter box and turbo (FYI this is not your intake pipe as I thought for ages!) it’s about 4-5 inches in size and has an electrical connector plugged into it, the connector comes out but pushing the little plastic clip down, not pulling it up!! Cost about £30 ‘ish’
2. Faulty DV (Dump Valve) changed this as its a possible cause and for £10-£15 its a no-brainer if it was the fault, it wasn’t but its looks newer so hey :-) This is the little black circular unit at the back right of the engine on top under the engine cover, it has 3 pipes into it and it about 2 inches in size. Plus on this a lot of people use Baileys dump valves to hear the cool noise, that’s very bad on the 1.8T's as these engines need to keep the Turbo pressure to work properly not let the pressure out to make cool noises, so if yours goes Wooooshhhhh as your change gear its either fooked anyway or your have a non-standard valve on, i would change it for an original DV as they are pretty good from what i have researched.
3. Faulty N75 Valve, maybe but it’s least likely of the causes from my research! This is a small 1" in size valve with 3 small pipes going into it and is located near the Dump Valve.
4. Faulty Lambda sensor(s) Before and just after the Catalytic Converter are 2 sensors, both tell the ECU (Car Brain) how much of each gas is in the exhaust, the front one, i.e. the one before the Cat is likely to go faulty telling the ECU to adjust the fueling to compensate for a non-existent problem such as over fueling hence it starts to over or under fuel and goes beyond the set tolerance values and trips the light on your dash :( this is highly likely as a cause. They cost £120ish each, you should replace them both at the same time but you would probably get away with replacing the front one only. Easy (ish) to do yourself if you can get to it as it’s just a screw type unit screwed into the exhaust before the CAT and that connects by a wire to the ECU. My advice if it is this is DONT buy cheap, I know VAG are expensive but EBay is not always your friend here!! That’s my advice!
5. the most likely cause is a Boost leak; this took me ages to get my head around so here is my understanding. You have 2 pipes going to the turbo on your engine, one from the intercooler on the front of the car and one from the air intake box. The one you need to look at is the intercooler one and the associated small pipes that feed off it, these are the most likely cause as if they leak the system tries to compensate for the loss in pressure and the outcome is lack of power etc and the codes in this post. It’s a bitch of a job but that’s why most garages can’t be arsed with it. Patience and lots of tea; I’m planning on taking the intake manifold apart this weekend, nothing I don’t have to, only what I need to do in order to check the pipes, then onto the main pipes between Intercooler and turbo and hope to find something obvious although a boost leak can sometimes only show up when its under pressure which can only be achieved on a rolling road as you cant boost by revving it! L
If anyone can add to this please do as this is my non expert DIY Phil opinion only although it is based on heaps of research and swearing at the car!!
Cheers and hope this all helps!
Phil (Feeling the credit crunch in my fuel bill) :-(