So here is my problem. I have a 2003 f150 with the 4.6 w and my problem started out with when driving 55 to 65 mph and decelerating I heard backfiring or popping noises from the intake. I replaced numerous vacuum hoses. the back fire occurs only when maintaining my speed, deceleration or stopping. the backfire does not always occur when driving and I cannot notice it as bad in the mornings until parking. I have replaced all plugs and fuel filter with no change, I ve noticed it only back fires when I let of the gas and it does not always happen and is a lot worse in the afternoon, but clears at higher rpms/speeds. then I cleaned the iac but decided to go ahead and replace it and also replaced a leaking erg. I got an iac from advanced auto and upon startup I can hear howling and idled at 1900 in park, when i unpluged it it would stop howling and idle would drop to 700 to 800 rpm. plugged back in and test drove howled at low speed and stopping and it began to shift hard, my idle was jumping when stopping and when stopped the truck would lunge forward and ide would jump up and down. got home and opened the hood and coolant showed signs of overflow but was not over flowing or boiling, level was about two inches below the full mark. Coolant appeared normal with now signs of fuel or fuel smell in it like it would if I had a bad head gasket. Because of the howling and high idle I put `the old iac back on and all symptoms I was having with the new iac were gone but then the next day with the old iac back on my truck was shifting hard with constant high rpms around 2000 and delayed shifting and lunging forward when stopped. any suggestions or guidance would be greatly appreciated.

 

It sounds like you may be running lean due to a vacuum leak somewhere. The PCV valve and hose assembly are a common issue. The rubber 90's on the ends of the PCV soften up and crack. It can also be a dirty or bad MAF. The backfiring could be from an exhaust leak.
The first thing I would do is scan for codes. If you do not have a scan tool, most AutoZones do it for free. Get the codes and post them here. We can then try to diagnose this thing.

 

Don't rule out a TPS sensor either.