One problem related to gas recirculation valve (egr valve) has been reported for the 1997 Ford E-150. The most recently reported issues are listed below.
1) engine begins to runs rough. ) egr ports become clogged 3) intake has to be removed and ports cleaned and injectors flushed. Was told by the mechanic that this is a known design flaw with the Ford 6 cyl. Engine, and that there is nothing that can be done to prevent it. It will have to be done again periodically.