ACFT HAD BEEN FUELED FOR APRX 4 HRS ENDURANCE. TWO HRS INTO FLT ON IFR FLT PLAN, PLT DESCENDED TO 3000 FT WITH INTERMITTENT VISUAL GROUND CONTACT. SHORTLY THEREAFTER, WITH THE ACFT ABOUT 10 MINS FROM DESTINATION, THE ENGINE BEGAN TO SPUTTER. PLT STATED HE SWITCHED FUEL SELECTOR FROM 'BOTH' TO UNSPECIFIED TANK, SET MIXTURE RICH AND PUMPED THROTTLE. ENGINE REGAINED PWR MOMENTARILY, SPUTTERED AND QUIT. PLT REQUESTED AND RECEIVED RADAR VECTORS TO DESTINATION AIRPORT, BUT INSUFFICIENT ALTITUDE REMAINED FOR GLIDE TO AIRPORT. FORCED LANDING EXECUTED AT NIGHT ON ROADWAY. INVESTIGATION SHOWED FUEL STAIN FROM LEFT TANK FILLER CAP TO TRAILING EDGE OF WING. REFUELING PERSONNEL AND ON-SCENE FAA INSPECTOR REPORTED DIFFICULTY IN SEATING FUEL CAP PROPERLY. AD 83-13-01 NOT COMPLIED WITH BUT VISUAL INSPECTION DID NOT INDICATE A NEED FOR REPAIR ON LEFT FUEL CAP. NO FUEL WAS FOUND ON BOARD THE ACFT. FUEL GAGES FUNCTIONALLY CHECKED AND FOUND TO OPERATE NORMALLY.