my truck threw a p0506 and the mech said the wiring going to the (correct me if im wrong) IAC (just to the left of the throttle body 2004 4.6) was only getting 3 volts, my truck has a slightly rough idle but everything else is working fine...my conlcusion, bad alternator...any other ideas?

 

Why are you guessing alternator?
http://www.obd-codes.com/p0506

 

The 3 volt reading seen by the mechanic might be a normal value depending on prevailing conditions when the reading was made. The IAC is fed a PWM signal, Pulse Width Modulated, and the 3 volt reading, if done with a conventional volt meter, could be the average sent to the IAC from the PCM. If the IAC signal is monitored with an oscilloscope, then the voltage would appear as a string of 12-17 volt square waves of various widths.

Certainly the alternator, faulty or not, would not cause a rough idle.

 

any other ideas then on that code? i did the Gotts mod and then the CEL came on

 

ah, i spoke before clicking that link...after the gotts mod and a new air filter, this code makes perfect sense, going to check for a vacuum leak and make sure the pvc from the gotts mod is good...btw, whats a good way to check for a vacuum leak?

 

I check for vac leaks with an unlit propane torch. Point the propane flow toward the suspected leaks. If there is a leak, the idle will increase while the propane is being sucked into the engine. Don't flood the engine bay with gas, but open and close the bottle while focusing on specific connections. If you are not comfortable using the gas, carb cleaner or the like will work too.

The 04'-05' trucks do seem to be more sensitive to changes in the intake tract, particularly increased air flow from modified or cold air intakes.