CREW OF AN SF340B IN CRUISE EXPERIENCED HEADACHES; NAUSEA; AND A SMELL OF AUTOMOBILE EXHAUST. CREW DONNED OXYGEN MASKS AND SHUT DOWN RIGHT ENG BLEED. AFTER 10 MINS; THE EXHAUST SMELL DISSIPATED.

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DURING CRUISE FLIGHT I; THE CAPT; BEGIN TO EXPERIENCE THE NECESSITY TO COUGH A LOT. I QUESTIONED THE FO AS TO HOW HE WAS FEELING. I EXPRESSED THAT I HAD A HEADACHE AND WAS FEELING LIKE I WANTED TO VOMIT. THE FO EXPRESSED THE SAME FEELINGS. I TOLD HIM TO USE THE OXYGEN MASK AND THAT I WOULD DO THE SAME UNTIL WE FIGURED OUT WHY WE WERE FEELING THIS WAY. THERE WAS AN ODOR IN THE COCKPIT THAT COULD BE COMPARED TO THE EXHAUST OF AN AUTOMOBILE. WE DID NOT ADVISE ATC DUE TO THE UNCERTAINTY OF WHAT WAS GOING ON. WE WERE QUITE CONFUSED. I HAD SUGGESTED THAT WE SHUT OFF THE LP BLEED AIR FROM THE RIGHT ENGINE SINCE THAT WAS WHERE THE MAJORITY OF THE AIR SUPPLY TO THE COCKPIT WAS COMING FROM. I DID THIS KNOWING THAT THE AIRCRAFT WOULD REMAIN PRESSURIZED WITH THE LEFT ENGINE LP BLEED. ABOUT 10 MINUTES AFTER SHUTTING OFF THE RIGHT ENGINE LP BLEED WE NOTICED THE EXHAUST SMELL DISSIPATE. THE REST OF THE FLIGHT WAS UNEVENTFUL. ONCE IN THE GATE AT ZZZ; I CONTACTED MAINT CONTROL ABOUT THE BAD SMELL AND WROTE THIS UP AS AN ISSUE. THE AIRCRAFT REMAINED DOWN IN ZZZ FOR A COUPLE DAYS FROM WHAT I UNDERSTAND. ONLY THROUGH HEARSAY I LEARNED THAT MAINT WAS VERY CONFUSED ABOUT THE WRITE-UP AND FINALLY WENT TO A LOCAL HARDWARE STORE TO BUY A CO DETECTOR. AFTER PLACING THE CO DETECTOR IN THE COCKPIT AND RUNNING THE ENGINE THE PURCHASED CO DETECTOR EXPERIENCED A CO ALARM REFLECTING THE PRESENCE OF CO IN THE COCKPIT. FROM THIS HEARSAY ABOUT THIS MAINTENANCE EXPERIENCE I CAN ONLY DEDUCE THAT THE FO AND MYSELF EXPERIENCED CO POISONING DURING CRUISE FLIGHT. THROUGH HEARSAY I LEARNING THAT THE ACFT WAS FERRIED BACK TO ZZZ1 BELOW 10;000 MSL WITH THE RIGHT LP BLEED LEFT CLOSED AND THE PRESSURIZATION OUTFLOW VALVES SEALED OPEN. I LATER LEARNED THAT THE RIGHT ENGINE WAS CHANGED OUT BECAUSE THE CAUSE OF THE CO COULD NOT BE DETERMINED.... LEARNED ALSO THROUGH HEARSAY.CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED HE AND THE FO FIRST NOTED THE NEED TO COUGH; HEADACHES; AND THE FEELING OF WANTING TO VOMIT. THE COCKPIT HAD THE CHARACTERISTIC ODOR OF AUTOMOBILE EXHAUST. THE CABIN ATTENDANT WAS QUESTIONED ABOUT CABIN AIR CONDITIONS AND ODOR OR SMELL AND NON E WAS NOTED. THE OXYGEN MASKS WERE PUT ON AND THE R ENG BLEED WAS TURNED OFF. MAINT WAS ADVISED OF THE INCIDENT AND THE LOGBOOK ENTRY DESCRIBED THE CONDITIONS EXPERIENCED BY THE FLT CREW. THE RPTR LATER LEARNED THE ACFT WAS WORKED ON THE NON-MAINT STATION BY TWO TECHS AND REMAINED OUT OF SERVICE FOR TWO DAYS. THE RPTR STATED THE ODOR WAS NOT THE ACRID SMELL OR SMOKE OF ENG OR APU OIL. THE ENG WAS A GE CT-7-9B.