You could have a dirty throttle body, an air leak or any one of a dozen common issues and a fault code read should help lead to it but it may need VAG specific equipment such as VCDS an old style timing light and four gas as these are a half way house into modern technology so require someone who has experience of both methods. They are quite a fussy engine to set up and don’t like even suffering a flat battery.
Thank you for reply. I have an Icarsoft Vaws diagnostic tool. Sometimes I get this fault code: 00516 lower adaption limit(additive) not re 00516, closed throttle (idle) position switch (F60), if I scan the car after a while, it is gone, and back again later..
The ADZ uses throttle body single point injection with a cable operated throttle and separate external idle control stepper motor with an integrated position potentiometer in the TB, the pot is factory set and must not be removed or you will never get it back where it needs to be. The throttle body with pot is VAG number 051 133 028 P (almost entirely specific to the ABS, AAM and ADZ auto) and WAS £969.77 but is now obsolete mainstream and only available from VW Classic.
If it is the positioning motor 048 133 031 then these are still listed from VAG UK at £298.14 and are easily available on the aftermarket from various companies although Bosch no longer offer their 0 132 008 601 which was the original make.