A LINE MECHANIC REPORTS HIS CONCERN ABOUT BOTH PILOTS LEAVING THE COCKPIT WHILE HE WAS PERFORMING A REQUIRED ENG RUN WITH CABIN CREW AND A FULL LOAD OF PASSENGERS ONBOARD AN AIRBUS A319.

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AFTER CHANGING A COCKPIT TRIM AIR VALVE; AN ENG RUN WAS REQUIRED TO VERIFY THE FIX. I WENT UP TO THE COCKPIT WHERE I FOUND CAPT AND FO IN THEIR SEATS. I TOLD THEM WE REPLACED THE VALVE AND WOULD NEED TO RUN THE ENG. THE PLANE WAS ALREADY LOADED AND CAPT JUMPED OUT OF HIS SEAT AND STATED HE WOULD NOT RUN THE ENG. I SAID I WOULD. I SAT DOWN AND STARTED TO SETUP THE COCKPIT FOR THE RUN. FO WAS STILL IN HIS SEAT. JUST PRIOR TO ENG START FO GOT UP AND WAS STANDING BEHIND ME OR SO I THOUGHT. I STARTED THE ENG AND WHEN I TURNED AROUND TO TELL FO THE VALVE WAS GOOD HE WAS NOT THERE. FO HAD LEFT THE COCKPIT WITHOUT SAYING HE WOULD NOT BE THERE FOR THE ENG RUN. I IMMEDIATELY SHUT DOWN THE ENG AND FOUND BOTH PLTS OUT IN THE JETBRIDGE. WHEN I QUESTIONED ABOUT WHY THEY LEFT THEY SAID IT WASN'T THEIR RESPONSIBILITY TO RUN THE ENG; AND I STATED 'MAYBE NOT BUT YOU KNEW I WAS GOING TO RUN THE ENG AND YOU WALKED OFF THE ACFT KNOWING YOU WERE SETTING ME UP.' HE SHRUGGED HIS SHOULDERS AND WALKED OFF. I FEEL WHEN FO WALKED OFF WITHOUT ADVISING ME; HE WAS SETTING ME UP TO MAKE A POINT TO THE COMPANY. MORE IMPORTANTLY I THINK HE COMPROMISED THE SAFETY OF ALL THE PAX AND CREW ONBOARD WHOM ASSUME THEY WERE IN A SAFE ENVIRONMENT WHILE THEY WERE ONBOARD. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: REPORTER STATED HE FELT THE FIRST OFFICER SHOULD HAVE AT LEAST REMAINED IN THE VICINITY OF THE COCKPIT DURING THE ENGINE RUN; IN CASE SOME EVENT REQUIRING AN EMERGENCY PROCEDURE INVOLVING PASSENGERS OR CABIN CREW MIGHT HAPPEN. REPORTER STATED HE WAS TOLD LATER; HE SHOULD HAVE FORMALLY ASKED THE FO; IF THE FO WAS WILLING TO ACCEPT RESPONSIBILITY OF THE CABIN CREW AND PASSENGERS DURING THE ENGINE RUN. IF THE FO REFUSED; THEN REPORTER WOULD BE REQUIRED TO HAVE THE ENTIRE CABIN OFF-LOADED.