I started my van one morning and it ran terrible. The motor idled rough, but seemed to even out at higher RPM. I originally got a code of P0303.
I did a complete tune up(plugs, cap, rotor, pcv valve,air filter). This did not correct the problem. I now got codes of P0300, P0303, P0306. I checked all electical connections. The one thing I did discover is when removing plug wires one at a time while the van is running, cylinders 3 and 4 made no difference in the way the motor ran. I pulled the plugs out one at a time to se if I had any spark, of which they did. To make a long story short, I have replaced the PCM, cam sensor, crank sensor, EGR valve, both Oxygen sensors. I now get a code of P0304. I have performed compression tests, which are also fine. I have given up, can you help!!!

look at the coil see if the metal windings are rusty and starting to seperate if so replaice. When in this condition it will still fire but not as intended.