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A) There is a misfire Technical Service Bulletin #99-020 dated June 7 1999 regarding a re-flash on the ECM.
B) These vehicles are also very sensitive to the spark plug selection. I recommend ONLY NGK PFR5G-11.
C) These injectors are prone to bleeding down and leaking fuel into the cylinders causing misfires, mainly when cold.
D) Also the fuel pressure regulator fails and leaks fuel into the vacuum line.
E) Corrosion under the coil can cause a poor ground and the coil can cause a misfire.
All common causes of misfire issues on this year model. That said, I do not think they are going to cure the misfire, but they should be checked/eliminated.
I am going to assume that this vehicle has a direct misfire in #4 cyl. that is always present. Please let me know if that is not the case, as you did not indicate this on your question, only the fact that P0304 is stored, and the plug is black. Is the spark plug back and wet, or black and dry?
Based on the fact that the spark plug is black indicates that there is fuel present, but it is not being ignited. If you have carried out all the steps listed properly I suspect the Ignition Control Module is failed.
First I suggest you inspect the wires/terminals at the ignition module, and coil. Also check for corrosion under the coil. You could also “pin out” and test the wires from, the Ignition module to the coil, or simply replace the Ignition module.
Let me know if you have any further comments or questions.
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