so i have a 96 f150 with the 4.9L and im finally having some issues with passing emissions in illinois my truck is popping p0171 and p0174 codes now since it has happened ive replaced my maf sensor, some bad vacume lines, egr gasket and according to my scanner when i drive my o2 sensors read fine and when i spray my motor down with carb clean there is no rise in idle so what can i be missing?

 

Have you inspected/verified the vacuum reservoir(s) are intact?

 

where would i find that

 

There is a large reservoir on the passenger side fender well. Most of them are made of metal, it looks like a large juice can. Larger trucks used a plastic version with dual tanks. The metal version tends to rust through on the bottom side. No amount of spraying carb. cleaner will ever find it....

There is also a much smaller one on the HVAC blower case that is easily overlooked. My old '92 F350 had a cracked reservoir that was a bear to find.

The other area to look at is your fuel pressure. If it is too low you will run lean. Your 4.9L equipped truck should have around 45 PSI with the truck running at idle. The pressure should rise to ~60 PSI with the vacuum hose disconnected. The pressure rises as the vacuum lowers (pushing open the throttle).

 

Yep,

Cleaning off all grounds (especially the block ground at the starter) may help the OP's situation too.

Resistive ground circuit(s) for the fuel pump(s) will cause the pump(s) to be "laboring" to produce the above required pressures.

Just my thoughts.

Bob