AN EMB145 FLT CREW ON THE GND EXPERIENCED FUMES IN THE CABIN WHEN THEY STARTED THE APU FOR PACK AIR. PAX WERE DEPLANED; AND THE CREW FLEW THE ACFT FOR MAINT; EVEN THOUGH THE PROB WAS IDENTED AS A FUEL LEAK.

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I WAS THE PF FOR THE LEG. UPON COMPLETION OF MY WALKAROUND AFTER ARRIVING. I FOUND NO DEFECTS AND THE ACFT WAS IN GOOD SHAPE; THE APU AND ITS HOUSING WERE ALL NORMAL; NO LEAKS WERE VISIBLE AND THE APU WAS OFF BECAUSE WE WERE ATTACHED TO GND PWR. WE LOADED THE ACFT WITH THE PAX AND CARGO; RECEIVED THE ELECTRONIC WT AND BAL SHEET; EVERYTHING CHKED OUT OK. ALL APPROPRIATE CHKLISTS HAD BEEN RUN AND WE STARTED THE APU. THE TEMP WAS NICE IN THE ACFT SO WE LEFT THE PACKS OFF. ONCE THE DOOR HAD BEEN SHUT AND THE BRIDGE HAD BEEN PULLED AWAY; WE WERE INSTRUCTED TO RELEASE THE PARKING BRAKE. AT THAT TIME I SELECTED PACK #1 ON. FUMES IMMEDIATELY FILLED THE ACFT (BOTH COCKPIT AND CABIN AS CONFIRMED BY THE FLT ATTENDANT) WHICH WE DETERMINED AS EITHER EXHAUST FROM ANOTHER ACFT OR FUEL. I TURNED THE PACK OFF; THEN WE ASKED THE GND CREW IF THEY SAW ANY ACFT BEHIND US. THEY SAID NO. I THEN TURNED THE #2 PACK ON AND HAD THE SAME HORRIBLE SMELL AND TURNED IT BACK OFF. WE THEN ASKED THE GND CREW IF THERE WAS ANY FLUID LEAKING FROM THE ACFT. THEY TOOK A LOOK AND SAID IT LOOKED LIKE OIL WAS POURING OUT OF THE APU. THE SMELL HAD GOTTEN PRETTY BAD IN THE PLANE SO I TOLD THE GND CREW TO GET THE PEOPLE OFF AS QUICK AND AS SAFELY AS POSSIBLE AND MADE A PA TO THE PEOPLE SAYING THE SAME THING. MEANWHILE; THE CAPT WAS COMMUNICATING WITH THE GND CREW AND MAINT. THE GND CREW REATTACHED THE BRIDGE; THE FLT ATTENDANT OPENED THE MAIN DOOR AND SVC DOOR TO HELP VENTILATE AND THE PAX WERE REMOVED. I JUMPED OUT AND WENT TO THE BACK OF THE ACFT AND SAW A DARK FLUID POURING FROM EVERY DRAIN HOLE AND STARTER INLET AND OUTLET IN THE APU COMPARTMENT. THE DARK FLUID TURNED OUT TO BE FUEL DARKENED BY GREASE AND OIL THAT WAS IN THE COMPARTMENT. THE APU WAS SHUT DOWN AND WE HAD THE GND CREW CALL ARPT OPS TO RPT THE FUEL SPILL. MAINT CTL HAD CONTRACT MAINT COME OUT AND MEL THE APU BY CLOSING THE FUEL SHUTOFF VALVE IN THE R WING AND COLLARING A CIRCUIT BREAKER IN THE COCKPIT. WE WERE THEN TOLD BY MAINT CTL AND DISPATCH TO OPERATE THE FLT BACK TO ZZZ1 AS OUR ORIGINAL FLT NUMBER EVEN THOUGH WE HAD NO PAX. UPON LNDG IN ZZZ1 AND PARKING AT THE GATE; ONE OF OUR MECHS TOLD US THAT BECAUSE IT WAS A FUEL LEAK ISSUE; THEY WOULD NOT HAVE ALLOWED US TO FLY BACK TO ZZZ1 AND WOULD HAVE GNDED THE PLANE. AS FAR AS WE KNOW; THE PAPERWORK WAS COMPLETED PROPERLY; THE MEL WAS SIGNED OFF WITH A MAINT CTL AUTH NUMBER FOR THE CONTRACT MECH; AND THE APU PANEL WAS PLACARDED WITH THE APPROPRIATE MEL AND ALL THE STEPS IN THE MEL WERE COMPLIED WITH. I BELIEVE THE EVENT OCCURRED BECAUSE A FUEL LINE BROKE IN THE APU COMPARTMENT. THE MECHS SAID THEY COULDN'T ISOLATE THE LEAK. AS THE CREW; WE FOLLOWED ALL COMPANY AND FAA POLICIES; PROCS; RULES; AND REGS. I DON'T LIKE THE FACT THAT MAINT CTL SAID FLY; AND MECHS SAID THEY WOULD HAVE GNDED IT FOR THE SAME SITUATION.