FUEL CAP MISSING. LAND ACCOUNT FUEL USE EXCESSIVE.

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RENTED THIS AIRPLANE FROM AN FBO AT MILWAUKEE; WI. FUEL FILL POINT WAS TAPED OVER BECAUSE FUEL CAP WAS MISSING. OPERATOR TOLD ME THAT THIS WAS ALRIGHT BECAUSE ACFT HAS AN INTERNAL VALVE AT FILL POINT THAT IS SPRING LOADED IN THE CLOSED POS AND WOULD PREVENT ANY FUEL FROM SIPHONING OUT. THE ACFT HAD BEEN FLOWN MANY TIMES IN THIS CONFIGN BY THIS EXPERIENCED OPERATOR. DEPARTED ON A TRIP FROM MKE TO PKB. 45 MINS OUT OF MKE; AT LEVEL FT; NOTICED AN ABNORMAL AMOUNT OF FUEL CONSUMPTION ON FUEL GAUGE. AS A PRECAUTION; TOLD CHICAGO CENTER THAT I WOULD LIKE TO LAND AT SBN BECAUSE OF A FUEL LOSS PROBLEM. LANDED SAFELY AT SBJ WITH 1 HR OF FUEL LEFT. SHOULD HAVE HAD AT LEAST 3 PLUS 45 REMAINING. FIRE DEPT WAS STANDING BY AND FOUND NO NOTICEABLE LEAKS; FOUND TAPE WAS PARTIALLY MISSING AND INTERNAL VALVE STUCK IN A PARTIALLY OPENED POS. FUELED ACFT AND DETERMINED FUEL SIPHONING HAD OCCURRED. I RELIED ON OPERATOR'S OPINION BECAUSE OF HIS EXPERIENCE IN THIS ACFT AND QUALIFICATIONS (ATP/CFI). THE INTERNAL VALVE SHOULD NOT BE RELIED UPON AND OTHER OPERATORS SHOULD BE ADVISED OF THIS.