My wife drives a 2010 Impreza auto with a EJ20 fitted. On the first start of the day it runs rough like it's only firing on one cylinder. After between 15 -45 seconds it suddenly comes good and operates normally. Following this first start and warm up the car operates normally for the remainder of the day even if left to stand for 5-6 hrs

I have returned the car to subaru dealer who changed the in tank filter assembly (not sure why)

I have cleaned mass air flow sensor and upper engine manifold clean with pressure pack + added injector cleaner with no result.

I am planning to remove throttle body and clean as next step.

Has anybody experienced a simmilar problem.

Also my OBD 2 reader will not recognise this car can anybody recomend a good reader as an alternative?

I appreciate any suggestions or advice

How old are the spark plugs and wires?

are you beginning to get cool/damp mornings ? bad plug wires could be an issue - they arc when wet, then dry-out.

 

(but, not sure if the 2.0 has coil-on-plug ?)

 

could be oil in the plug tubes I guess?