I have ruled out vacuum as a problem by removing the pipes of the solenoid and joining the pipes together this closes the exhaust flap.

I have scoped the solenoid on both wires and have 13.9 volts on both wires with no pwm signal

There is no need to drive it as im told it should close when the car is started.

I think i'm going to call it a day on this one

The customer only brought it in because the old back box has corroded through

Spending a couple more hours labour diagnosing a fault that is only going to make his car a little quieter while idling is not top on his priorities but I said I would enquire on here on his behalf

thanks for the suggestions

Matthew ladds