Oscillating Boost

Today I experienced an interesting problem with my 9000 Aero.

It has the Speedparts Stg 3 fitted which gives quite an increase in boost pressure – often off the end of the red section of the gauge.

While on a drive today I started to make use of this boost, and the first vigorous acceleration was fine.

After this, I started to experience boost oscillations – it might make it to the red, then drop back to the start of the yellow, red again, yellow and repeat. All this while the throttle is held steady, and accompanied by significant changes in the car’s acceleration.

Everything else about the car is fine – I checked all hoses and they seem ok. It drives normally but this only happens when I try high boost levels.

My guess at what could be causing this is the dump valve….

It is the standard fit and so 8 years old. If there is a problem with the diaphragm then the boost could build to a certain level, the diapragm fails, boost drops, diaphragm becomes ok, boost builds again and repeat.

However faero from Saabscene kindly lent me a spare dump valve, and this didn;t fix the problem, which lead me to suspect APC valve.

Now I tried disconnected the pipe from the APC to the wastegate.

Result – Lots of boost.

Used the in-dash gauge, and the needle went rapidly over to the horizontal, way past the end of the red.

There was also a strange high-pitched noise, so I only did this a couple of times and very briefly.

Reconnected the APC valve, and back to boost fluctuations.

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