THE ENGINE LOST PWR AS THE AIRPLANE WAS RETURNING FROM A FIRE-PATROL FLT. THE PLT MADE A FORCED LDG IN A FIELD WHERE THE AIRPLANE RAN INTO SOFT DIRT AND NOSED OVER. POST-CRASH EXAM OF THE ACFT REVEALED NO EVIDENCE OF ANY FUEL RAMAINING IN THE FUEL SYSTEM. THE EXAM ALSO REVEALED THAT THE LEFT TANK FUEL CAP GASKET WAS MISSING AND THAT A LARGE FUEL STAIN EXISTED IN THE AREA OF THE LEFT TANK FILLER OPENING. THE PLT RPTD THAT HE HAD DEPARTED W/ FULL TANKS WHICH HE HAD VISUALLY CONFIRMED. HE INITIALLY OPERATED ON THE LT TANK FOR 1.5 HRS THEN SWITCHED TO THE RT TANK FOR 2.1 HRS. WHEN THE RT TANK RAN DRY HE SWITCHED BACK TO THE LT TANK BUT THE ENGINE DID NOT REGAIN PWR. THE PLT NOTED THAT AFTER PRIOR PATROL FLTS LASTING ABOUT 3.5 HRS, THE ACFT WOULD HAVE 5-6 GALLONS OF FUEL REMAINING. HE NORMALLY ESTIMATED FUEL REMAINING BY THE KNOWN AVG CONSUMPTION RATE OF THE ENGINE AND ELAPSED TIME. HE DID NOT GO BY FUEL GAUGE INDICATIONS BECAUSE THE GAUGES WEREN'T 'THAT ACCURATE'.