1998 f150 Idling erratically. 125k miles. 4.2, Manual. At a stop my truck will idle between 500-800RPM. With the A/C on it will dip down into the 200-300RPM range then bounce back up, and idles much rougher. It just bounces back and forth like its dancing to the damn radio. I changed the air intake controller module, or whatever its called, (looks like a cigar, sits on top of the engine). Changed the fuel, air filter, and its still dancing. The lip on the neck of the fuel filler pipe has a small crack in it. Could this be causing it? I thought that that seal was just to keep the fumes out of the ozone, and served no other purpose. Thanks.

 

Start looking for vacuum leaks. Inspect the PCV tubing very closely, especially at the rubber elbows for any breaks that'll allow outside air into the engine, causing your weird idle.