I have a 2003 f150 with the 4.6 w and When driving 55 to 65 mph and decelerating I hear backfiring or popping noises from the intake. I have replaced numerous vacuum hoses. It is weird because I only have the back firing occur in the afternoons when decelerating or after putting it in park. the engine has a slight rough idle but nothing to bad. One more thing the idle does drop down, raise, drop down then back to normal idle when coming to a stop, such a stop light, but not to the point of a stalling. Please help!!

 

How many miles on the truck, when was the last time the spark plugs were replaced? Have you ever cleaned the throttle body and IAC?

 

the truck has 215000 on it but i had to replace the head gasket about a year ago, but I went ahead and changed every gasket, cleaned, and added new parts while I had the top half tore down! the plugs probably need to be changed been around a year or less. I have cleaned both the throttle body and IAC when I replace the head gasket. intake back fire occurs only when maintaining my speed, deceleration or stopping. the backfire does not always occur when driving and I cannot notice it in the mornings until parking. Thank you

 

There is really nothing solid to go on.
Check for codes first.
This is a good case for using a Scanner to look at live data to detect what's out of common operating limits.
Backfire is associated with timing the fuel and spark event.
Only way popping in the intake can occur is if the spark fires with the intake valve open even a small amount and/or fuel injector adds fuel to the problem.
During decell. the fuel is supposed to be shut down and the IAC closed down.
If there is an issue with any of these in the right combination the pop could be the result.
Add to this a possible timing chain/cam timing issue and you have the possible causes of the problem. When you had the top tore down are you sure the chains were timed correctly?
At 215k you have wear that likely is adding to the issue.
Good luck with it.

 

the increase and decrease in idle at a stop sounds like a throttle position sensor to me, not sure if it would have anything to do with the back firing tho

 

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 mostly happens in the afternoon, but clears at higher rpms/speeds. I have also noticed that when I left off the gas the rpms are slow to fall, is this normal or should the rpms drop as soon as I left off the gas? And sometimes when I come to a stop I hear a whistle or howl when coming to a stop again this does not always happen. When I changed the plugs they are looked good nothing abnormal. also when I turn off the ac I hear bubbling or gurgling sound in the cab. Any ideas what is going on??????

 

The plugs and fuel filter have nothing to do with the cause of the backfire.
I explained the Deceleration action involving the IAC.
Again, the Deceleration action has to close down the IAC and cut back the fuel or you can have the issue.
You have to investigate those actions for a fault.
First the IAC to be sure it's not gummed up and slow to respond.
If it does not close fast the motor will 'hang on' to rpm longer when you lift off the throttle.
Good luck.

 

Ok so I cleaned the iac but decided to go ahead and replace it I 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 stopping the truck would lunge forward and ide would jump up and down. got home and opened the hood and coolant showed signs of overflow, 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. I thinking it could be a leaking intake gasket or bad valve. any suggestions or guidance would be greatly appreciated. I also put `the old iac back on and all symptoms I was having with the new iac were gone.