I've got a 99 forester with a 2.5 sohc that started acting up. It started sputtering and popping while I was driving it and no codes showed up. When it does it I can mash the peddle to the floor and it takes off. So I guessed and put a new fuel pump in her, but the same condition is still present. Funny thing is it's idling fine and when you give it some throttle it responds. It's only when I'm driving at lower Rpm between 1500 and 3000.

Have you replaced the fuel filter. Sounds like that's the problem.

If it drives OK when you "mash" it, then I don't think that the fuel-filter is blocked.

It could be either the Throttle Position Sensor TPS, or the Idle Air Control Valve IACV.

checked the crank/cam timing? maybe slipped a tooth

 

knock sensors are known to kill power without setting a code - always worth checking on older soobs. Many folks do fine with cheap ebay replacements.

 

yours may be cracked - usually a bad sign

Kinda sounds like Bad MAF sensor.

Have you replaced the fuel filter. Sounds like that's the problem.

 

^^ +1 on this. Always best to put in a new one especially if you replace your fuel pump.