Audi a4 18T tubo model 2007 why does it mis only when it idils

It could be a few things, but my guess is a fouled spark plug. This could also be a sign of a bad ignition coil, clogged air filter or something wrong with the air sensor, or a major issue like a leaky head gasket. Start with the easy stuff like the air filter and the spark plugs. Also, try to borrow or pay someone to read the fault coded with a VAG-COM, at the very least it will tell you which cylinder is misfiring.

Check for any air leaks on the inlet system.these models uses quite a few small rubber vacuum hoses for various functions like manifold flaps, fuel pressure regulator and turbo actuator and dump valve. any air that is sucked in by the engine after the airmass meter is known as false or unmeasured air, resulting in a lean mixture at low revs. get some carb cleaner and spray it all over the inlet system and injector seals while the car is running.if there is a leak, the idling speed will change when it sucks in the carb cleaner.