fuel vapor problem on 1991 Ford Ranger

tried smoging ford ranger 4x4 4.0
but said I couldnt past fuel vapor pressure line test...what is the usaul deal with this? help please thanks

1 answer
1996 model years and onward the computer in the car runs an evaporative emission test automatically by closing solenoids and measuring how long it took pressure or vacuum to decay. By doing this it could calculate if there was a leak in the evap system that was allowing harmful ozone depleting pollutants into the atmosphere. Older cars didn't have this self test feature so the state was worried about the harmful effect older cars had, so they require on smog tests that the technician fill the evap system with an inert no explosive gas to check for leaks in the gasoline vapor storage system. Could be as simple as a bad gas cap , bad rubber hose or bad hose clamp or something more complex. A repair shop would need a "smoke machine" to look for the source of the vapor leak.