hi i have been having a power loss hesitation issue on my forester 12/97 model ej20 sohc engine.
i have replaced coil pack as cyl 2 had a weak spark diagnosed by all the other spark plugs being burned like you would expect for their full service lifetime but number two looking new still (not rounded spark burned electrodes) this improved it but didn't fix the issue . I then tested fuel pressure and replaced all the injectors with a known good set and replaced the very old O2 sensor (350,000 kms ) with a new one again a small improvement but the issue still persisted some times working perfectly but most of the time hesitating on acceleration and lacking power and not maintaining a constant speed. I then checked valve clearances and compression the middle 4 valves on both cylinder heads were were a little tight on the clearance but this is normal as they run slightly hotter than the outside valves again no change . The problem turned out to be the electronic relay driver box at the back rhs of the engine it has the two large plugs with all the engine wiring going through it and it contains the high currant driver electronics for the injectors and coil drivers turns out that one of the two capacitors that are part of the coil driver circuit were getting to the end of their life and making an intermittently weak spark on cyls 1 and 2 i have heard many others report a similar problem that persists after changing all manner of other sensors, ignition components and even the main ecu. I only found it by a process of elimination and knowing that older cars with points ignition often behave like this from a bad condenser, this sent me looking for the coil driver circuit . the spark was not weak on voltage but lacking heat form the blown capacitor. since replacing this driver box with a second hand one the car now runs really smooth and goes up hills in fifth where it used to stutter up it in third and fuel consumption has improved from 9 km per l to around 14 km/l I hope this helps others :-)
Edited by scoobydoobie, 23 August 2014 - 07:52 AM.