Have a 2002 Alero with 3400 series engine. Recently changed plugs, wires and coil packs as car was running a little off and check engine light was coming on and code P0301 was being thrown. Car seemed to be all good but now when you start the car after it's been sitting for a long period say overnight or when leaving work the car starts fine and idles fine for about a minute then the idle stumbles very badly for about 10 seconds like it's going to stall and then it surges to around 2000 rpms for several seconds and then settles down to normal. Once in a while was this is happening the check engine will flash a couple times and go out and once in a while it will stay on and the code given when it stays on is once again P0301. This is the only time you get a code and no code is given when the light only flashes and goes out. The car runs fine and gas mileage is normal. This only happens when the car has been sitting for extended periods of time and the engine is cold.

 

Replace #1 injector

 

I've never done any injector work before, but I do alot of my own work. How hard of a job is this to do on this engine? Also what exactly is happening that causes the issues at a cold startup only and why does the car still run so good and get good gas mileage? Is there any other possibilties as well?