A PA23 AZTEC PLT DIVERTS TO CHA; TN; WHEN HE NOTES FUEL SIPHONING FROM THE L INBOARD FILLER CAP. PLT HAD RPTED A SLIGHT OVERSHOOT ON HIS CLBOUT WHILE NOTING THIS LOSS OF FUEL.

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I WAS ON AN IFR FLT IN A PIPER TURBO AZTEC F FROM A PVT STRIP IN 33GA TO BNA ON DEC/XA/98 AT APPROX XA30. CEILINGS ON DEP WERE ABOUT 800 FT AGL WITH LIGHT RAIN AND MIST. LIGHT RIM ICE WAS ENCOUNTERED IN THE CLB. TOPS WERE ABOUT 8000-10000 FT MSL. APPROX 15 MINS INTO THE FLT AND AS I WAS TRANSITIONING FROM CLB TO CRUISE AT MY ASSIGNED ALT OF 12000 FT MSL; I NOTICED A LARGE SPRAY OF FUEL LEAKING FROM THE LEADING EDGE OF THE FUEL CAP IN THE L INBOARD TANK. BEFORE THE LEAK I HAD 2 1/2 HRS OF FUEL -- ENOUGH TO FLY TO NASHVILLE; THEN TO MY ALTERNATE (MEMPHIS) PLUS 45 MINS RESERVE. ANY LOSS OF FUEL WOULD NOT ALLOW A SUFFICIENT RESERVE. I NOTIFIED ZTL OF THE SIT AND ASKED FOR VECTORS TO CHA WHERE I KNEW THAT THE WX WAS VFR (2500 FT 10 MI VISIBILITY). CHA WAS APPROX 20 MINS NW OF MY LOCATION. I ESTIMATED 45 MINS OF FUEL IN THE L OUTER WING TANK. I QUICKLY SWITCHED TO THE INBOARD TANK IN ORDER TO MAKE USE OF AS MUCH OF THE FUEL AS POSSIBLE BEFORE IT ALL LEAKED OUT. ATC SEVERAL TIMES OFFERED VECTORS TO NEARER ARPTS WHICH I DECLINED AS ALL WERE IN UNFAMILIAR HILLY TERRAIN WITH CEILINGS NEAR MINIMUMS. I DECIDED THAT THE WX; TERRAIN; AND GOOD ILS AT CHA WERE MUCH SAFER IN THE EVENT OF A SINGLE ENG LNDG. THE ONLY DEP FROM AN ASSIGNED ALT OR CLRNC CAME AS I WAS TRANSITIONING FROM CLB TO CRUISE. BECAUSE I WAS PREOCCUPIED WITH NOTICING THE LEAKING FUEL I OVERSHOT MY ASSIGNED CRUISE ALT BY ABOUT 220 FT. ATC PROVIDED EFFICIENT VECTORS FOR A VISUAL APCH AT CHA WITH A SAFE LNDG. A MECH LATER DETERMINED THAT THE LEAK WAS CAUSED BY A FAULTY O-RING IN THE FUEL CAP AND A LOOSE ADJUSTMENT OF THE LATCHING MECHANISM. NO FURTHER LEAKS HAVE BEEN EXPERIENCED SINCE REPLACEMENT OF THE O-RINGS AND ADJUSTMENT OF THE LATCHES. COMPARISON OF THE AMOUNT OF FUEL PUT IN THE TANKS BY THE LINE CREW WITH THE FUEL BURN FROM THE FUEL COMPUTER SUGGESTS A TOTAL LOSS OF ABOUT 5 GALLONS FROM THE L INBOARD TANK.