740 turbo 1989. First start in mornings stall,runs for a minute and stalls. Gas milage is lousy. Things that are changed,O2 sensors, spark plugs and wires,turbo intercooler hose, clean throttle body, clean fuel pressure regulator with carb cleaner, clean idle air control motor.After these were done the engine idles up and down at 800-1400rpm in park but doen't stalls.When in drive it idles at 700-750rpm.( still lousy gas milage).Does oxygen sensor or oxygen sensor fuse have anything to do with the bad milage and up and down RPM?. Can you tell me where is the engine diagnostic socket located and how to check if there is any fault codes or how do you know if there are codes in the system? I am kindly asking for any help. Thank you in advance your help would be greatful. MOE