I have a 2000 XLT with the 4.0 SOHC V6. The engine sounds as though it is pinging when it is under moderate to hard acceleration when the engine is cold. If I speed up very slowly it will not ping. It only does this when it is cold. Most cars I have owned only did this once the engine reaches operating temperature. This one pings while cold and stops pinging once it reaches operating temp??? I have tried tightening the lower intake and I have cleaned the MAF twice. I did find a stuck thermostat this evening and replaced it, but when I drove into work this evening it started out pinging and continued to do so until the engine warmed up. I was in the process of taking a look at the ECT when I noticed that the truck had been running for almost 20 minutes and the upper radiator hose was ice cold. I then noticed that the anti-freeze in the radiator was also still cold. My best guess was that the ECT was probably getting bad information because there wasn't any anti-freeze flowing past it. It will probably take a day or two to see if I am on the right track. Also, where is the IAT sensor? Haynes says it should be just down stream from my MAF. It is not there, nor is there a fitting for one. That was going to be my next stop. Thanks in advance for any help you can give me.

 

My 99 x has the same problem with pinging when its cold, and I have never been able to found a solution yet. If you find the answer please let me know.
As for the IAT, I believe it's built in with MAF. That is where I found mine. You will need a special security star tool to access it.