Car stalling while driving

2002 I DONT KNOW THE CODE OFF HAND BUT IT WAS A MULTIPLE MISSFIRE CODE…WHEN WE CHANGED THE COIL PACKS THAT WENT AWAY SO ITS NOT HAVING ANY CODES NOW…IM NOT SURE ON THE FUEL PRESSURE HOW DO I CHECK THAT…THE CAR HAS ABOUT 5 GALLONS OF GAS IN IT NOW