Out of ideas and need help. I have a 1992 Wrangler YJ 4.0 that after sitting for about 7 years, I cannot find the engine knock cause. It sounds like a knock coming out of #2 cylinder. The cause is not from push rods, rocker arms, lifters, rod bearing, main bearing, no vacuum leak, bad sensor, and properly lubricates. It does not knock when first started in cold weather for about 30 seconds. It knocks 90% of the time- bad knock. While troubleshooting I removed the valve cover to see if there might be a flattened cam lobe or bad valve, but when I took the cover off I could not hear it and everything appeared to be operating correctly. Nothing seems to be striking the valve cover. I feel like I have a either a piston slap or need a valve job. At this point if I have a valve job, it would be cheaper to buy a rebuilt head. While in there I could replace the pistons, but if I am wrong I just threw away money if it doesn't fix the problem. I have found a rebuilt engine that after returning the core would cost $1500 not counting installation costs. That's too much on my budget, but can't afford to be wrong. Has anyone else ran into this problem or have any ideas? Don't know enough about floating valves, but my dad used it to pull out a tractor that may have caused the problem.