This is incredibly helpful, thanks a lot, Tom.
You're very welcome. I'm glad it's helped.
Given what you say, is there any use in me asking a garage to see if they can 'clean up' or 'check the electrics' of my existing ASV (manifold flap 'throttle body', as it seems to be called by the parts people), in the hope that they discover it doesn't need replacing after all (and can therefore eliminate the EML warning light)?
Or would that attempted cost-saving effort be a total waste of time - just better get on and buy the new part and be done with it?
As the car's driving perfectly OK (and has just got through its MOT), do you think any further work on this can safely be postponed until I've saved up some more cash?
The term 'throttle body' is somewhat inaccurate in this case, though I understand why people use the term. A throttle body is a part of the air intake system that controls the amount of air flowing into the engine. It's basically a butterfly valve that can introduce a variable obstruction into the pipework, thus regulating the airflow. In a petrol engine, where the amount of air fed into the cylinders needs to be precisely controlled, the butterfly valve can be moved to any position to finely adjust the extent to which the flow of air is obstructed.
Our TDI engines run 'wide open'. In other words, there's no need to regulate the flow of air into the engine; maximum air is always OK. As such, there is no throttle body. However, when you switch the engine off, it's best to starve the engine of both fuel and air. Starving the engine of air at shut-off is performed by the anti-shudder valve, which is basically a throttle body that only has two positions: completely open or completely closed. Finely controlled, intermediate positions aren't required.
When driving, having a failed anti-shudder valve is no problem. The anti-shudder valve only performs its job when you switch off the ignition. You'll find that your engine shudders to a halt rather than stopping smoothly.
The major problem with a failed anti-shudder valve is that it's also there as a safety net. Let's imagine that your turbo seals failed, causing hot oil to be allowed into the engine's air supply. If your car isn't able to block the air supply at shut-off (due to a failed ASV), then the engine would continue to be fed both the ingredients it needs to run: air and fuel (the hot oil!). You'd have no control over the engine and it could rev itself to death. As such, when shutting off an engine with a failed ASV, you should depress the clutch, engage 5th gear and be ready to lift the clutch and stall the engine should it continue to run after you've switched off the ignition.
I'm yet to look into refurbishment possibilities for the electronic anti-shudder valve fitted to the BHC and ATL engines, but I'm sure it's possible. I imagine you can pick up a replacement from a breaker with relative ease and fitting is a quick job.
Let me know if I can be of any further help. I'm not too far from you.
Cheers,
Tom
