Ok well here is the deal.... I have a 2000 F150, 2wd, 5-speed with the 4.2. It has the Airforce One intake, Magnaflow Cat back and a super chip.
Its been running great up until a couple days ago. Now when you are some what lugging it say when you are going 25 or so in 3rd gear or right at 35 in 4th gear and give it the slightest little bit of gas it (just lightly accelerating) it makes a slight pinging sound. for about 2-3 seconds while you accelerate. It runs great the performance is all there and it only makes the sound in that exact same situation.
So I reset the ECU just to see. No change. I then Take the chip out to see if maybe I got some bad gas and that was affecting it. No change. So I take it to the dealer. They drive it and put it on the scope and says that there are no codes and everything checks out. The feel as though the intake could be affecting it. I told them it has worked great until a couple days ago. They then tried to tell me I may need to run premium with the intake. I tell them that I had just filled up on premium. And now Im really schocked when he tells me that I should not run anything more than 87 octane that it could be harmful to the motor. I basically tell him hes crazy. They then say that the injectors could be dirting or that the throttle body could be opening improperly. I ok them to check out the injectors and clean them as well as replace the fuel filter.
Ok, $238.00 later it still makes the sound. I dont even make it out of the parking lot and I go back. I tell them its still doing it. They then recommend me to run the gas thats in it out and bring it back with 87 octane in it?
So now Im stumped. What else could it be? They said all the plugs were firing properly. Could it be bad gas? Could the chip have hurt the ECU? Anybody else see anything like this?

Thanks for the time guys!

 

heh.. welcome to the club. i have a 2000, 4.2L 2wd automatic. do a search for "knock" or "knocking" on the forum and you should find some good posts. ive been going through somewhat the same thing. tried different octances/octane booster, cleaned the MAF, reset the computer, put brand new plugs, etc. next thing im probably gonna try is check the throttle body and then seafoam the engine. seems a lot of people on here have had luck using seafoam to clean out carbon deposits and eliminate or reduce the knock.

BTW.. does it sound worse when the engine's cold? i know mine sounds horrible driving down the street from my place on the way to work in the morning, but after hopping on the freeway and driving to work, the knock is still there, but it doesnt sound as bad. and yes, just like you, it only happens when accelerating, even lightly. mine is a bit worse though.. it stays with me through acceleration, even in 1st. only really goes away at high RPM or freeway speeds (55+).

 

Same here. I've found carbon build up to be a severe problem with these engines, as the EGR and PCV systems are very effective. The PCV system tends to suck oil as well as crankcase gases throught the intake, coating the intake runners in oil. This oil makes its way into the cylinders and creates carbon build up. Upon using a cleaner, some carbon is sucked throught EGR system and eventually back into the intake.

As an experiment I attached a clear fuel filter to the PCV tube at the throttle body. I was trying to reduce the oil consumption, but found that more that just oil is sucked through there. Small peices of black carbon are speckled around the fuel filter. I think spraying a good cleaner throught the intake is needed to clear off all the gunk.