This could be an airleak after the airflow meter, any freshen dragged in here isn t seen by the ECU and therefore not compensated for and can lean the engine out and can afterward allow the engine to rev going on in the manner of not desired causing scratchy running. As its a mechanical idiosyncrasy it tends not to turn on the oddity lively and you can sometimes listen a hissing noise similar to the engine running.
Its afterward worth getting the fuel pressure checked as if this is low due to a blocked filter or faulty regulator or even a ill pump will every upshot in insufficient fuel physical delivered to the engine. Might moreover be worth cleaning out the idle enthusiasm run motor / valve as these get clogged up following carbon and some brake cleaner washed through helps free things off.
This could be a large vacuum leak ,with the MAF amalgamated it is by yourself measuring the ventilate that is getting bearing in mind the MAF, in view of that if the intake manifold is sucking in air the let breathe to fuel ratio will be certainly out and cause the engine to stall. If MAF is disconnected the engine goes into a limp measurement and lets the engine run. later than carb cleaner spray can spray can in the place of the hissing later the engine handing out and subsequently you listen the engine stall or rev forward-looking there is the leak ,could be a bad vacuum hose bad gasket on the manifold.