This could be an airleak after the airflow meter, any let breathe dragged in here isn t seen by the ECU and as a result not compensated for and can lean the engine out and can plus allow the engine to rev going on once not desired causing gruff running. As its a mechanical aberration it tends not to turn on the malformation vivacious and you can sometimes listen a hissing noise in the manner of the engine running.
Its in addition to worth getting the fuel pressure checked as if this is low due to a blocked filter or faulty regulator or even a below par pump will all upshot in insufficient fuel inborn delivered to the engine. Might furthermore be worth cleaning out the idle eagerness direct motor / valve as these get clogged stirring like carbon and some brake cleaner washed through helps free things off.
in the same way as I begin the car, the engine surges happening in the middle of 1200 and 2000 rpm assist and forth. I changed the IAC and the TPS past no tweak in symptoms. I have noticed that several Lexus SC's have this same issue. Prior to this I would have to get used to the idle readiness screw to belittle the rpm and it would maintain for a while. Now, the engine is for ever and a day surging to the lessening where I cannot drive it for bell of broken to the drive train. Any experienced solution?