Ok, I have a bit more of an update.

Since last weekend I have not heard my relay click when the car is cold. Also since I pulled off the vacuum hose to the turbo and sucked on it, I and felt the mechanism working, the car was pretty good till yesterday. I had to pop out at lunchtime to buy something in town, and as I drove down the road at work the car stuttered a few times. Car was around 2500 rpm in 2nd (iirc). After that the felt did not feel as nippy as it has been earlier on in the week. Driving home last night, I could not hear the normal whistle noise which I normally get reflected back at me with the window open, and accelerating was lacking (but not true limp home mode).

Therefore I am wondering if maybe I have a slightly sticky turbo? It was changed around 60k miles ago, and most days the car gets a reasonable work out. I am wondering if come the summer when it is warmer and I have a bit more time on my hands if I should take the thing off and have a go at cleaning it?

Next time I drive it, I'll see if the relay clips come back - I am wondering if they only happen when the vanes on the turbo get stuck?

Maybe it's sticking so that you aren't getting enough boost rather than too much. So the vanes would be in the position it would normally be in when going full chat and that would cause it to have a lot more lag at lower revs... probably...

I dunno quite how they work, I assume the vane adjustment is at the same angle for a given rpm, regardless of what gear I am in. So yes, I wonder if the thing is stuck in a low boost position.

The stutter when it happens seems to be around 2500rpm, and the relay click when I first used to hear it (pre stutter) was a bit lower 2300rpm. I am sure that at lunchtime yesterday there was no relay clicking going on.

Pesky motor!

I'm not sure either. Is there an electronic control on the variable nozzle, or is it driven purely mechanically?

As far as I know it is purely mechnical, with the actuator being controlled by vacuum from whatever the valve is.

n75 valve, but what controls that valve? The magical ECU I guess?!

n75 valve, but what controls that valve? The magical ECU I guess?!

Voodoo!

Voodoo!

Given my car, that is a pretty good possibility

I know the N75 has an electrical connection, but I don't know if the connection drives the valve, or supplies data to the ECU!

n75 valve, but what controls that valve? The magical ECU I guess?!

Yep. The engine ECU operates the electrical supply to the N75, and that controls vacuum supply to the turbo actuator, which alters the vane angle.

Where does the vacuum come from? Maybe that is the problem, or would a fault be logged if that was too high/low?

Vacuum comes from a plastic ball at the front of the engine IIRC.

FWIW, it sounds like a stalling turbo, but in theory you shouldn't get that from a VNT turbo on a TDI.

As a quick update to this, I pulled off the vacuum supply hose from the n75 valve and have sucked/blown down it. I can see the mechnical vacuum arm in the turn moving, and I can feel it with ny hand too (once I have stuck my hand down the back of the engine). Obviously I have no idea how hard I am mean to need to stuck to get movement, but I little old me can do it, so that might point away from sticking vanes in the turbo and more towards something else??! Just off to drive to a mates so I will see if the car is feeling more lively than it drive back on Thursday evening (after the lunchtime stutters).

While I was in a sucking/blowing mood I also pulled off my two wiper washer fan jets and ensured they were clear too, as the drivers side one has only been squirting on one side of the fan (towards the nearside). Infact I swapped them from side to side as well.

IIRC (and we're going back to when I was 11) most people can generate 5 or 6 PSI of suck or blow (which is why you can't quite use a straw to get a drink from a bottle on the ground out a first floor window, and yes we did make a 3m long straw and try it!) and I think that's similar to the level of vacumn depression you get on the atmospheric side of an inlet turbine in a turbo-supercharger unit.

As soon as I dropve down the road with the window open I could hear the turbo noise again.... so maybe the vanes are getting stuck? I did scan earlier, and there were not faults logs (overboast), so maybe it is getting stuck in an underboost position, and my manual sucking is freeing it up?

It's unusual, but I honestly think it's not impossible (for reasoning, see above).