99 f150 with 5.4...has 207000 miles, replaced altenator, water pump, sparkplugs, and one coil. Now it idles low and dies unless I keep my foot on the accelerator not sure what it is?

 

The IAC stuck shut or somethings blocking the intake line to the IAC.

 

To remove the Alternator you need to remove the air inlet tubing. Did you tighten all the clamps and reinstall the two tubes going into the adapter just in front of the throttle body? Are they loose allowing a leak. Follow them back to the valve cover and the IAC to be sure they did not get knocked off.

JMC