Could be lots of reasons for that--first thing to check for is any leaks,you risk a fire if its drooling gas somewhere even for a short time after a cold start..
I assume your truck has fuel injection being an 87,and anything that old may have a old flexible fuel hose or steel line that has degraded and lets some fuel seep out under pressure..
There is a charcoal canister under the hood that collects fuel vapors,sometimes they get saturated with raw gas or fail to vent properly,that may be a cause also..