2002 Sport with 4.0 (97000 miles).....Kept getting OBD II 300, 301 & 302 codes. Eventually, the engine light began flashing and I quit driving. My local shop recommended changing the coil rail. The cost for them to do it was outrageous so I bought one and changed it myself. As long as I had it apart I went ahead and ran compression on all 6 cylinders (152-155 psi on all) which I thought was good and also installed new spark plugs. When I started it up, I could really tell the difference it how it sounded, and a road test showed a marked improvement in power and performance. I drove it for about 25 miles, stopped for awhile, and upon starting up again, the check engine light came on showing a 301 code. I didn't notice any engine miss at all. Any suggestions on what I should check next?