I wanted to share an interesting and surprising discovery with a timing problem that exhibited all your symptoms and thought it worth sharing as it was something that you wouldn't have thought of.
I purchased an Impreza that has an EJ204 with AVCS timing and would intermittently suffer terrible acceleration, rough idle but if you pressed the accelerator slowly, it would gradually accelerate as opposed to virtually nothing if you tried to accelerate quickly. After swapping out many sensors (MAF, IAC, Oxygen, TPS, knock) without success I too was running out of options. The isolation of the knock sensor or an ECU reset improved the situation for a period of time, then go back to the intermittent poor accelarion. I thought it may be fuel pressure or timing.
One thing I did notice was at cold start, a funny sounding exhaust note and always a slightly harsh vibration of the engine at idle, warm or cold even without this other problem.
I hooked up a software based scan tool and like every one else, no fault codes whatsoever and no real clues. Timing was however a little erratic and sometimes would jump around with the knock detection. Isolating the knock sensor made a difference but I had replaced this also. The timing indicated 15 degrees at warm temperature but when put in drive with the brakes on to load the engine, it would bounce around a bit and certainly give some backfires and was retarding the ignition. The only thing I did notice was the advance /retard timing of the right hand bank of cams was very slightly different to the left hand bank (this side has the cam pulley sensor) After some deep thinking, I decided to go right back to basics and check the cambelt auto tensioner and timing.
After removing the covers and crank shaft pulley and checking the timing, to my surprise, the right hand bank of cams was one tooth out on the timing! On top of that the hydraulic auto tensioner was not exerting enough force which compounded the problem of erratic timing. I am certain it had not jumped a tooth.
According to a label under the bonnet, the Cambelt had been replaced about 50,000k ago so looks like it was not timed correctly. The interesting thing to note is that the timing is derived from the left hand bank of cams (has the cam shaft pulley sensor) and so the ECU would not pick this problem up. In addition, because the engine has the AVCS, it was trying to compensate for this and if I ran lower octane fuel (91 instead of 95) the probelm would show up worse but most of the time would run ok.
Well, after, setting the timing correctly, replacing the cambelt auto tensioner, the engine engine idle is super smooth and the intermittent poor acceleration problem has been eliminated. Also the funny sounding ehaust note has disappeared. I was able to put the knock sensor back to original as with all the other sensors! The scanner also showed that the AVCS timing between the two banks of cams is now very similar.
I had spent many hours on this problem and rather than keep going round in circles, I thought ouside the square and decided to go back to basics. Glad I did as otherwise this problem would never have been found. It had been to two different mechanics and they could not find the cause. The previous owner did 40,000k and I had done 10,000k before it began to show up as a real problem. This experience may help someone else with one of these intermittent poor acceleartion issues.
