While collecting parts for the project, in my cheap as possible style I brought a second hand VAG diverter valve to act as the blow off valve. My reasoning was that I would just divert the boost pressure to atmosphere rather than back into the intake, as VAG intended for this valve.

However, the valve didn’t work as expected. The first problem was that on idle (high inlet vacuum and no boost pressure) the diverter valve would open, sucking in unfiltered air from behind the radiator. Not ideal.

The second issue was that the valve didn’t seem to open when it was supposed to (coming off the throttle on high boost) this would cause the turbo compressor to 'surge'. This is bad for the turbo, also it makes a very loud 90's boy racer noise 'zoot zoot zoot' noise. Initially i loved this but I quickly grew tired of it.

I reasoned that perhaps the vacuum source I was picking up from was unreliable (vac source for the inlet manifold actuator) and tapped into the line for the brake servo. This made no change.

I thought that perhaps the preload spring on the diverter valve diaphragm was too strong. After market items all seem to have adjustable springs. I dismantled the valve which wasn’t easy (all moulded plastic) with the intention of trimming the down and found a split in the diaphragm.

New valve ordered (part number 06A145710P) and it worked flawlessly. The 'zoot zoot zoot' noise was now replaced with a loud 'whoosh' noise. At least the compressor wasn’t surging risking damage to the turbo.

I fitted a small air filter to the outlet of the valve to filter air on idle. This also had the effect of silencing the 'whoosh' noise to a much more acceptable level. I suppose the filter is also 'self-cleaning' too!