OBD II, ('96 and up) ZJ's have a "self learning" computer. Power programers, tunes, and other devices that modify the stock settings on how the engine runs will work for a very short time, but the the computer will learn around it and go back to factory spec. Any gains you had, will be gone. Also timing can not be adjusted in any way. Timing is read in real-time from cam position sensor and crank position sensors.
Sorry to be the one to give you the bad news, but theres no easy way to get power out of a OBD II 4.0 inline 6 without bolt-on upgrades.
If you want to, look into the 703 injector swap, '99+ intake manifold swap, and try to get the exhaust to flow better. From there, it gets complicated, but you could upgrade the head and cam or port polish the heads to get better flow, but all that is a bit of work that a lot of people will not attempt
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