Toyota Repair: 4-runner engine ping, combustion temperatures, coolant temp

Question
I have a 99 4-runner 3.4 V6 automatic.  It pings (spark knocks) under acceleration.  It only does this when it is completely warmed up.  There is no check engine light, however there was a P0171 code that came up earlier.  I replaced the MAF, the coolant temp. sensor, full tune-up and rear 02 sensor(bad wire) and this temporarily fixed it.  I've ran cleaners to clean out carbon also.  Nothing seems to fix it, it always comes back.  Mileage was @24 now is more normal @ 21.  The light however doesn't come back on but the problem is getting worse. High octane seems to be a temporary fix as well. Please help if you can, this thing is driving me nuts.

Answer
With this preignition pinging going on there should be a check engine light on, the PO 171 code indicates a lean mixture condition which is normally caused by the MAF, since it was already replaced it can't be the cause, an EGR malfunction can cause the combustion temperatures to be higher causing preignition but if that was the case there should be a check light indicating that it is the case, there are a couple of possibilities that may cause this and not set a trouble code to make the check engine light come on, there may be a problem with the computer itself although not common, the engine has knock sensors which detect pinging and when they do they send this information to the computer which then retards the timing but only up to a point but again if the sensors can't adjust the computer and the pinging persists the check engine light will come on, without being able to do active tests where I could see real time data when the pinging occurs I can't be of too much help, I would recommend that you have the toyota dealer make a diagnosis, sorry but this problem will take a technician that knows what they are doing with the proper equipment to diagnose, keep me posted.