I drive a 99 Grand Prix GT, just under 90k miles on it. About a week and a half ago, I left for my friend's house on the other side of town. On my way there, I noticed a misfire/sputter, and the SES light came on. By the time I got there, it was misfiring big time, and wouldn't even idle right. So the next day, I installed new plugs, wires, and coils. Misfire went away, SES light went out, and everything seemed to be running tip top. About 3 days later, I was cruising around, and it started a small misfire again and the SES light came back on. Took it to Auto Zone, and it had a random misfire code again.
Now, it starts fine, idles fine, but when I go anywhere and tach above 2400-2500 rpm, it starts to misfire. I changed the fuel filter. Changed the oil, it looked fine, no coolant present or anything like that. Checked for vacuum leaks, didn't find anything. Checked the fuel pressure, and im only getting 40 lbs. which is a little low according to specs. I'm thinking it is the fuel pressure regulator, or the fuel pump. I'm not too savvy when it comes to the fuel delivery system.
So, my question is, would a bad regulator or fuel pump cause the pressure to be slightly low like this and cause a random misfire? I've been working on this thing for days, and I'm getting nowhere fast. Any help would be greatly appreciated.
