A C177 PLT BECOMING DISTR WHILE FUELING HIS ACFT; NEGLECTED TO REPLACE THE FUEL CAP; RESULTING IN AN EMER LNDG AT THE NEAREST ARPT.

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I WAS PREFLTING MY CESSNA CARDINAL FOR A 4.5 HR FLT FROM THE WEST COAST TO BOISE. DURING THE REFUELING; I WAS INTERRUPTED BY MY PAX (WIFE) TO ASSIST HER LOADING LUGGAGE. I COMPLETED REFUELING BY COLLAPSING THE LADDER AND SECURING THE FUEL PUMPS. 1.5 HRS INTO THE FLT; I WAS AT 10000 FT OVER THE CASCADES MOUNTAINS WHEN MY WIFE NOTICED THAT THE R FUEL GAUGE WAS INDICATING 1/4 TANK; AND THE L FUEL GAUGE WAS INDICATING 1/2 TANK. TO MY HORROR; I REALIZED THAT I HAD NOT REPLACED THE FUEL CAP AFTER LOADING THE BAGGAGE; AND THAT I PROBABLY DID NOT HAVE ENOUGH FUEL TO LAND AT THE NEAREST COMMUNITY ARPT E OF THE CASCADES. I PUSHED THE EMER GPS BUTTON FOR THE NEAREST ARPT TO LEARN THAT A 1400 FT GRASS AIR STRIP WAS UNDER ME IN A NARROW MOUNTAIN PASS. I IMMEDIATELY BEGAN A DSCNT; HOWEVER; I WAS UNABLE TO REACH AIR RADIO DUE TO THE SURROUNDING HIGH TERRAIN. THE LNDG WAS BUMPY; BUT UNEVENTFUL. AS I STEPPED OUT OF THE ACFT; I WAS SOAKED WITH FUEL DRAINING OFF THE WING. TO MY RELIEF; THE FUEL CAP WAS STILL HANGING FROM ITS CHAIN. I MEASURED THE REMAINING FUEL; REPLACED THE FUEL CAP AND CONTINUED MY FLT TO THE NEXT COMMUNITY ARPT. CONTRIBUTING FACTORS: I ALLOWED MYSELF TO BE DISTR FROM A CRITICAL PREFLT FUNCTION (REFUELING) AND I DID NOT COMPLETE A FINAL PREFLT CHK OF THE FUEL CAPS. I DID NOT MONITOR ENG AND FUEL GAUGES DURING FLT.