Hi, I'm finally about to begin looking for a VACUUM LEAK in my '99 E-150/4.2L/65K miles(bought new).
Actually, the symptoms are; AC Blend-Doors or something forcing AC air from defrost-to-floor-to-defrost with Vacuum change and speed change(should be a TSB/Recall).I hear a scratching noise from the dash during the AC air shift. It's been like that but functions for years. Also,my installed cruise-control(vac.assisted) does not acellerate with the same vac.speed push(like not enough vac).It does hold the speed steady enough.The van runs fine(did plugs & wires at 50K),but seems to lack some acelleration.The tail-pipe shows black soot(seems common),thought maybe the exhaust/cats are getting plugged? The plugs at 50K did not look too sooty. Now that the van is older, I'm hesitant to start pulling rubber/plastic vacuum lines to various connections. I've seen posts regarding some invisible Elbow Hose under the throttle body(?) that's prone to leaking.I'd appreciate any suggestions as to how to begin the search and testing of the E150 Vacuum system. Also periodically, I get a PO455 CEL,fuel vapor, which I scan & erase. Could that be tied into the Vacuum system?What does the shoe-box size Vacuum Reservoir Tank which sits on the frame-rail(passenger side)do exactly(other than almost fall off the van)? Thanks for the help.
Ron

 

FORD has usually published 'Electrical & Vacuum Troubleshooting Manuals'. If other year manuals are as helpful as the 1 I have, I'd strongly recommend seeking a copy for '99 E150. They turn up on ebay or order thru Helm. FTE Sponsors may stock them.

 

Ron, I have a V-10 in my van and had that hose elbow deteriate under the throttle body. I found the leak using a mechanics stethoscope, you could actually hear the leak. You have to remove the air intake system to reach it. Vacuum leaks could be anywhere though.
Good luck and let us know what you find. John