850 T5 Boost issues

Hi, hopefully someone can help me, my 850 T5 is behaving very strangely when pushed.
In any gear, if you put your foot right down, the boost needle goes off the end of the gauge, the car shudders and 'coughs then the boost drops off, if you keep your foot down it shoots back up then does the same thing until you back off the throttle.
The guy I bought the car from didn't mention any mapping being done but I don't remember my old auto ever boosting over about 3/4 up the gauge, could it be to do with that, it has a large cone filter instead of airbox too. I am at a loss and it is doing my head in, I can only drive at half throttle which is ok for normal driving but obviously no fun.

Have you checked the Vac line to the actuator? Also the one off the bottom of the compressor housing.

Yep - sounds like boost-cut.

If it's got a bleed valve on it, shut it down. if not, Like Merc says, check all the vac pipes before you bend something important.

Thanks guys I will check, not sure about a bleed valve but will check

it shouldnt be boosting that much even if it is mapped, are the pipes to the actuator on?

Quote:

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Originally Posted by T5frankie

it shouldnt be boosting that much even if it is mapped, are the pipes to the actuator on?

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I did think it was strange, gonna have a look when I get a chance, not driving it till its sorted