To start, my son has a 2000 XJ 4.0 with 3"lift and cut fenders. He bought it used and yes, it had the dreaded head that was cracked, so being a jeep lover, I replaced the head with a 0630 casting out of a junk yard 97 ZJ. The XJ ran rough and kept having a P0302 (misfire cylinder 2). I changed the plugs, coil rail and verified the timing. This did not work, so I did a cylinder pressure test, all were within tolerances but #2 was the lowest pressure so I changed the rings in all cylinders. Still idled bad and threw the P0302 code again. So I bought a remanufactured head from NC heads and just replaced it (again) no code (yet) but idles like crap almost like it is missing again. I tested the coil rail again with a Mac coil tested and the fuel injectors with a Mac fuel injector tester.
So to summarize, new head, new cylinder rings, new plugs, good coil rail, good injectors, timing is correct, no loose wiring, no cracked or leaking vacuum lines and no codes... Why is this idling rough?
I am at my wits end and out of ideas, can anyone help?
Before the head change, was the engine runnng rough like it is now?
I would expect the new head to have new valve springs. Did it?
I think the only thing that remained the same with the head juggling was the Rockers, pivots, bridges and push rods. Correct me if I'm wrong.
You may want to double check the rockers and pivots on #2 for excessive wear, the push rods on #2, and verify that the bridges aren't twisted. As a general practice, before torquing down the rocker capscrews, each respective cylinder should be at TDC compression to take the load off the bridges. Torqued, in steps, to 21 ft. lbs.
Even though you are not getting a fuel injector DTC, the injector for #2 may be clogged, or otherwise defective. Try swapping #2 injector with #3 and see if the problem follows. Ensure these injectors have new o-rings fitted before reinstalling.