87Truck

Have 1987 4X4 V6 D21 200,010 miles 5 speed. The truck keeps failing the Hydrocarbons test all other tests including CO and NOX pass no problem. Installed new plugs (NGK) cap, rotor, PCV valve and airfilter. Checked timing fixed 2 vacuum leaks. The truck uses oil about 1QT per 1500 miles but no sign of any smoke out of the tail pipe. I even checked the AIV and AIV case system everthing works. I also repaced the O2 sensor and checked the cylinder head temp sensor operation still no help. I would appreciate any advice. Thank you