2000 Jeep Cherokee problems after head gasket change

Engine Mechanical problem
2000 Jeep Cherokee 6 cyl Four Wheel Drive Automatic

I have a cousin who changed his head gasket on a 2000 jeep cherokee with the straight 6 cyl. It has the "distributerless" ignition system. After he finished and started it, the engine revs up to redline, almost like at full throttle. I went to help him and see what I could do, and sure enough it does immediatly rev right up to the redline and stays there. Thought it could be the idle air control valve, checked it with an obd machine and even said it, replaced it and still does it. Checked "very" thoroughly all vaccuum and emission lines, no leaks or cracks whatsoever ! Took the valve cover off and made sure all rocker arms, push rods and valves are moving correctly.I have never come across this before in all my years. He insists everything he did was done correctly, he is very keen on doing things slow and taking his time and checking everything twice. I would appreciate any help or suggestions. I am completely baffled and confused on this one. I am wondering at this point if it is a jeep thing and I just dont understand, but it would be nice if he could drive it again.