Where boost used to rise to 0.9bar and stay there it now rises to about 1.1 hold that for 1sec then drop to around 0.6bar then build back up to 1+ over a few seconds. This cycle then repeats.

Might be related to fitting a free-er flowing exhaust and cold weather? Can the map sensor fail so that ecu doesn't control boost then runs into knock?

Poor boost control. ie marginal overboost, then cutting back to boost, then overboost etc etc. Could need a restricter to sort it. Wait for one of the more techy guys to come along, like John Banks et al.

When did it start doing it and have you made any mods recently?

Matt :)

Changed the exhaust. Central cat removed so secondary turbo de-catted (its a tt legacy) standard downpipes then 2.5" back.

Also had an oil leak fixed and oil seems to have been overfilled - on hot-mark when its cold.

Duncan,

I'm not sure if this applies to B4s as well as single turbo cars but if the oil is overfilled it can contaminate the pipes to/from the boost solenoid, you need to reduce the amount of oil in the engine and clean the pipes with brake cleaner or the like.

Does your car have multiple solenoids? I have a mechanic friend who describes my car as a plumbers worst nightmare...he should see your car...

Andrew

Sounds like it could either be contaminated Boost piping with oil and/or the exhaust change.

Matt :)

More solenoids than you can shake a stick at. I was planning to clean / document all that gubbins anyway.

Most likely the overboost option - overfilling just flattens it rather than making it oscillate!

Overfilling can cause the piping to become blocked/partially blocked which means that the fuel cut can be hit due to the overboosting that can occur. Once the fuel is cut and the boost drops, it starts building again until it hits the cut.

This is the ocillation cycle I think is being discussed and can often be cured (on Imprezas at least) by cleaning the boost control piping.

Matt :)