I have a 1999 subaru impreza that will back firer when u try to accelerate has a check engine light for knock sensor but that has been changed any ideas will help

I'd have thought more likely to be Cam or crank sensor.

I do have a knock sensor code don't know if that will cause these problems 

Replace the knock sensor then.

I have one on order but the one in the car only 6 months old

could be one of various sensors, have you chagne any pipework recently?

No it all still the same apart from a wastegate gasket 

sounds like its hitting fuel cut, check for leaks in the pipes. if youd change the pipework i;d sugest restrictor pill, but if thats not changed then must be somethign else

I haven't changed anything apart from draining all the fuel as it was sitting for 12 months