A C172 PLT; ON A CROSS COUNTRY FLT; RAN OUT OF FUEL AND MADE A FORCED LNDG AT PAE.

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I WAS RETURNING FROM A TRIP TO BOZEMAN; MT (BZN); AND STOPPED AT WENATCHEE; WA (EAT); TO REFUEL; GET A BRIEF REST AND THE LATEST WX OVER THE CASCADES MOUNTAIN RANGE AND PORT ANGELES; (CLM). AFTER MY CESSNA 172 WAS TOPPED OFF; I DID THE CUSTOMARY PREFLT: VISUALLY CHECKING FOR FULL TANKS AND INSURING THAT THE FUEL CAPS WERE PROPERLY REPLACED. DRAINED FUEL FROM THE GASCOLATOR UNDER THE ENG AND VISUALLY NOTED THAT IT WAS NOT LEAKING. AFTER COMPLETING THE PREFLT; I BEGAN MY FLT FROM EAT TO CLM. INITIALLY; I CLBED TO 8500 FT TO REMAIN UNDER A 10000 FT OVERCAST. SINCE THAT ALT IS BELOW LCL PEAKS; I DID NOT BOTHER WITH FLT FOLLOWING AND WAS KEPT BUSY NAVIGATING WHILE CORRECTING FOR LCL SHIFTING WINDS. I WAS NOT CONCERNED ABOUT FUEL AT THAT TIME SINCE I HAD TAKEN OFF WITH A 5 HR SUPPLY FOR A 2 HR FLT. AS I PROCEEDED W; THE 10000 FT CLOUD LAYER DISAPPEARED AND I DECIDED TO AVOID THE CLASS B AIRSPACE. I CLBED TO 10500 FT AND PASSED DIRECTLY OVER PAINE FIELD (PAE) AND HEADED DIRECTLY TOWARD PORT TOWNSEND (0S9) IN ORDER TO CLR THE CLASS B AIRSPACE AND BEGIN MY DSCNT. I WAS DIRECTLY OVER PAE WHEN I SUDDENLY NOTICED THAT MY FUEL GAUGES WERE INDICATING ZERO FUEL. THE ENG CONTINUED TO RUN NORMALLY. I RECYCLED THE ELECTRICAL SYS; BUT THIS ACTION DID NOT SOLVE THE PROB. BY THIS TIME I WAS OVER THE S END OF WHIDBEY ISLAND AND I COULD SEE THE 0S9 ARPT (APPROX 15 NM AWAY). WHEN THE ENG QUIT AFTER 1.6 HRS; I TRIMMED FOR OPTIMUM GLIDE; BROADCAST A MAYDAY AND SUBSEQUENTLY MADE A SUCCESSFUL ENG OUT LNDG AT THE 0S9 ARPT. AFTER LNDG I CHECKED THE FUEL TANKS AND THEY WERE BONE DRY. THE NEXT DAY I PICKED UP MY MECHANIC AND WE DROVE BACK TO 0S9. IT WAS DETERMINED THAT FUEL HAD INDEED LEAKED FROM THE GASCOLATOR DRAIN VALVE DURING FLT EVEN THOUGH THE PREFLT ACTIVITY HAD NOT DISCOVERED ANY PROB. OBVIOUSLY; THE VALVE HAD SOMEHOW OPENED DURING THE FLT. WE REFUELED THE ACFT AND I FLEW IT TO CLM WHERE THE DRAIN VALVE WAS REMOVED AND INSPECTED. TWO PARTS AND ASSOCIATED SEALS WERE REPLACED AND THE ACFT RESTORED TO SVC. INITIAL POSITIONING OF THE PLUNGER IS BY RETURN SPRING PRESSURE. A CABLE CLAMP INDIRECTLY SECURES THE LOWER POS OF THE DRAIN VALVE PLUNGER. THE ACFT (AND PROBABLY THE GASCOLATOR DRAIN VALVE) IS 20 YRS OLD. THE MOLDED SEAL ON THE END OF THE DRAIN VALVE PLUNGER IS CONICAL SHAPED. OVER THE YRS THE SEAL HAS CHANGED ITS SHAPE. IN ORDER TO CONTINUE TO FUNCTION AS A SEAL; ANY SLACK IN THE ORIGINAL CABLE CLAMPING HAS PROBABLY LONG BEEN ELIMINATED. ALSO; THE LOWER PLUNGER POS RESULTS IN A REDUCED SPRING LOAD DUE TO THE LONGER TRAVEL TO THE CLOSED POS. IN EFFECT; TIME HAS CAUSED THE SPRING LOAD ON THE PLUNGER SEAL TO BE RELIEVED BY THE INITIAL CLAMP POS. I HAVE OWNED THIS ACFT APPROX 5 YRS. IT HAS OFTEN BEEN NECESSARY TO PUSH DOWN ON THE CABLE KNOB TO SHUT OFF THE GAS FLOW WHEN PERFORMING THE PREFLT. UNTIL NOW I WAS UNAWARE THAT THE VALVE SHOULD CLOSE AUTOMATICALLY WITHOUT AID. I'M SURE THAT I'M NOT THE ONLY ONE IGNORANT OF WHAT IS NORMAL OP FOR THIS FOR THIS VALVE (WHICH IS TO BE OPERATED EVERY PREFLT OR WHEN REFUELED). THE ACT OF PUSHING DOWN ON THE CABLE KNOB DOES SEAT THE PLUNGER AND THE 'HEAD' OF THE GASOLINE PLUS THE ATMOSPHERIC PRESSURE HELP IT TO REMAIN CLOSED. I CAN ONLY SPECULATE THAT WHEN I WENT FROM 1249 FT TO 10500 FT THAT DAY; THE DECREASING ATMOSPHERIC PRESSURE ALLOWED THE VALVE TO REOPEN AND DUMP FUEL OVERBOARD. WHY THIS DIDN'T HAPPEN THE DAY BEFORE WHILE I WAS FLYING OVER THE ROCKY MOUNTAINS AT 12000 FT I'LL NEVER KNOW. I'M JUST THANKFUL IT HAPPENED WHEN AND WHERE IT DID. RECOMMENDATIONS: 1) A SAFETY ALERT SHOULD BE WRITTEN FOR ALL GASCOLATOR EQUIPPED ACFT NOTIFYING ALL AFFECTED ACFT PLTS AND MECHANICS OF THIS LONG TERM SINGLE FAILURE CONDITION. SPECIFICALLY; PREVENTIVE MAINT SHOULD BE PERFORMED WHENEVER THE GASCOLATOR DRAIN VALVE DOES NOT OP SMOOTHLY AND AUTOMATICALLY TO STOP THE FLOW OF GASOLINE WHEN THE CABLE KNOB IS RELEASED. 2) A READJUSTMENT OF THE CABLE CLAMP MAY BE PRUDENT TO ADD TO THE ANNUAL INSPECTION TO RESET THE CABLE SLACK IN THECLOSED POS. I AM A RETIRED SYSTEMS ENGINEER WITH 38 YRS OF EXPERIENCE INCLUDING HYDRAULIC VALVE DESIGN AND RELIABILITY; MAINTAINABILITY AND SYS SAFETY ENGINEERING SPECIALTIES. WHILE EMPLOYED; I WAS A REGISTERED PROFESSIONAL SAFETY ENGINEER.