Service schedule for 1991 Econoline 7.3 V8
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#2
It would be in the diesel engine suppliment for the 7.3 usually with the owners manual. Try a search on ebay.
http://cgi.ebay.com/ebaymotors/1992-...mZ110300576103
http://cgi.ebay.com/ebaymotors/1992-...mZ110300576103