I have a 99 Gt 4.6. It had the check eng light on it when I bought it. The guy had taken the stock exhaust off. And had straight pipes with no cats. So the check eng light was no surprise. I found a stock exhaust on craigslist and put it on. It lost all of it's sound. And the check eng light came right back on. This time with the P0300 multiple misfire code. And misfire codes for all 8 cylinders. It is not missing on all 8 cylinders. Does anyone know what could be causing this code? I put new plugs. Gaped them. Checked every coil pack. Put new injectors. Fixed small exhaust leak. New fuel filter. New camshaft pos sensor. New map sensor. I'm running out of ideas. California smog laws suck. My car runs fine. But because of the stupid light I can't even drive it