Can someone please help? I have a 99 f-150 4x4, w/ 208,000 miles. runs great! problem is sevice engine light has been on for amost a year, code is p0401, egr flow insufficient. have changed egr valve, epde sensor, vac switch, both epde sensor hoses, removed throttle body and cleaned ports (2), removed and cleaned egr downtube, checked all gaskets and hoses for vac leaks. after each replacement have reset comp. w/ scan tool. have even taken to a dealer for diagnosis to no avail, after 10 miles service engine light comes back on, code p0401, It's drivin me nuts! I understand that in the downtube there is a disk w/ a small orifice between the 2 small tubes the hoses connect to, but when i cleaned the tube fluid seemed to flow right thru, was going to replace downtube but was told at ford dealer that could not be the problem. this ones got me stumped! any ideas from soeone smarter that me would be appreciated greatly! thanks