My gfs car (04 Sentra 1.8 SE 51,000m) is having issues starting. When starting it spits and sputters for a few seconds and then straightens out. It only does this when its cold. The service engine light has come on and we put it on the computer. The computer said it was misfiring on multiple cylinders. We changed the plugs and no improvement. I have read there are fuel pump problems with these cars. Could this possibly be the problem or could it be something else.