I have a 2001 f150 5.4 I have put a new intake manifold on it,replaced plugs and cops and put new elbows on the pcv hose it still runs the same as it did before a changed everything like crap I can still hear it sucking air somewhere behind the throttle body and can not find any vacuum leaks anywhere and it is throwing a p0171 and a p1504 iac ciruit code I recently changed the iac valve also and nothing has helped it any help would be much appreciated!

 

If I take you correctly, you did not justify the replacement of those parts.

The 1506 code is a 'circuit' issue not a sensor issue. It's not able to be controlled properly by the computer!
The 171 code could be either a vacuum leak into the intake tract or even an exhaust leak ahead of the bank 1 Ox sensor.
If an air leak on the exhaust manifold, the intake won't have any effect.
To check for this, use a mechanic stethoscope that can remove the probe and be used to 'hear' with. Probe around the manifold to head pipe clamp area.
An air leak in this area draws air in as the exhaust flows by the open area. The Ox Sensor will detect this and shift the long term fuel table rich setting the code 171.
Good luck.

 

Thank you for replying I cleared all the codes on it and the check engine light has came back on (note that it is still running like crap)but I put my scanner back on it and it it giving the p0171 code again, but that is the only code it is giving now,I am planning as soon as I get time to check it and make sure I don't have anything stuck under the new intake manifold that could be causing a leak just want to double check myself before I go spending more money in parts