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A leaky injector is also a possibility, maybe even more so then a failing sensor since the malfunction is during acceleration and not often on a steady speed. This afternoon I had values reaching -25%. Car was heavily shaking at that moment.
I checked the fuel regulator vacuum connection, that one's dry.

Space wrote:Yes I know. There is no connector near the sensor, just wiring with bends and curves (wiring seems too long) and some tape around it halfway. By looking at it I'd say the sensor may have been replaced in the past.
I'm at a workshop tomorrow for refilling my ac system. I'll ask them to take a look at the wiring. It will be easier with the car up.

That's cause for concern in itself!

I agree and that's why at this moment the repairman is taking a look at the wiring. It is not attached properly and kept too long (so unnecessary dents/curves in the loom).

good

We have an unused wire, brown/white. 5 wires from the loom are attached to the 5 wire sensor. That 6th one is not. Hmmm.

That sounds like a universal sensor that isnt appropriate to your car.

http://www.ebay.co.uk/itm/Front-lambda- ... 19da046fd4

Thats the one youll need - not a bad price either. The plug will need to be replaced where its missing, but they are only a few quid from VW

I fixed the micro switch for the trunk light. Works again! Thanks Alex. Also replaced the old battery and cleaned the ground connection on the engine. Can't reach the ones below the battery properly without taking the bumper off. Oh well...

So, our GTIs have a 6-wire wideband oxygen sensor? That's really rare. The 6th wire should be signal ground reference.
If I get the original sensor and the wiring colors are different from the ecu wiring, then I will still have a problem matching the wires.

I think its a 5 wire sensor - I dont think the 6 wire is correct for your car.

Ill check my GTI over the weekend for you

I took the car to an injection system specialist to get the injectors tested and cleaned. They were quite dirty but there was no serious problem found with them. The car now performs clearly better, it's quite a difference but it did NOT solve the juddering problems on acceleration.

Only thing left is the oxygen sensor or perhaps ECU. I think I'll order an original sensor and try to add the original connector to the wiring loom since it is currently soldered together.

Oh no, the Ebay link sais it's sold out...

Try this one

http://m.ebay.co.uk/itm/181573786848?nav=SEARCH

Thanks Alex!
The connector has 6 pins as you can see. The wiring from ecu contains 6 wires. So the wire/pin count match, but I have no idea what color goes on which pin. Do you have an electrical schematic of that connector with the wire colors? Then it will be a no brainer to restore the wiring, connector and sensor back to how it's supposed to.

Leave it with me. I'll dig out the wiring schematic for you. Pm me your email address

In the meantime I refilled the ac and continued working on my audio setup. I have put a lot of work into adding vibration dampening and absorption, and also taking preventative measures against rattles. The car is much quieter now inside. I changed the battery fusebox for a Lupo one since it has an extra fuse that I now use for a power cable. There's a keyhole shaped rubber near the pedalbox where it goes through the firewall. It runs alongside the original wiring loom towards the trunk. I added extra protective nylon sleeve to the cable. At the back of the rear seat, I have put MDF dressed in grey clothing. This looks great and easy for fastening the amplifiers onto and cable clips. I have an Alpine F405 for driving an Alpine swr-1042d in a closed housing. It makes the car shake and your intestines haha. Now still need to add a second amp for powering the door speakers since the head units internal amp just can't cope with the Alpine.