2000 Toyota Corolla ce Check Engine light.?

[fsrrii]

I have Toyota corolla 4cly 1.8l EFI DOHC I have had a check engine light for 5 months I took the MAF sensor out cleaned it worked for a couple of hours then came back on, I have changed the spark pluge twice and now the car is taking long for it to start i have to pump the gas paddle a couple times to get it to start i also changed the Throttle Position Sensor and it is now slow when you start to drive it. I can hear it back fire in the engine area when I am driving it these are the codes i got on 11/11/2009 p0302 p0171, I took my car to the dealer they charged me $49 for dignostics they couldent find the problem they asked me if they can do more dignoses on it for another $49 , i had to leave my car for the weekend to check it some more , they are gussing they already check few things , they want to drain the gas they think it might be bad gas, but i dont think so. also they are going to change the air filter box cabin he said there may be a leak, will have to wait tell monday and see. will it is now tuesday now they are saying it might be the all 4 fuel injectors and it will cost $1100 for the repair.