It fires,and starts, but wont idle? I was gonna say does it have oil, but obviously it does. Does it have enough to keep enough pressure on the oil pressure switch that turns on and off the fuel pump? If so, check a few things. Make sure you CONNECTED the mass airflow meter(i forgot to do this after I reassembled my M20 after doing a top end overhaul). If its connected, MAKE SURE the air intake boot is NOT ripped. If its ripped ANYWHERE in between the Mass airflow meter, and the throttle body, or the idle air valve and the mass airflow meter, it wont work properly. Also make sure the hose clamps(worm drive) are TIGHT so air dont leak in between the boot and the throttle body and/or the mass airflow meter. Id also check the intake manifold gaskets, throttle body gaskets, AND injector O rings. Check these by spraying with brake cleaner, and having someone else crank the engine. Also check the elbow bend for the idle air control valve for leaks to the back of the throttle body. If all that stuff is good, its possibly the idle air control valve. Its either stuck(fixable with solvent and brake cleaner) or if its burned up, they ARE NOT cheap to replace. They cost in between 450-600. I would ALSO check EVERY ground. Make sure it is PROPERLY grounded, coz if it is NOT, it can cause all kinds of WIERD problems. Grounds if not PROPERLY applied can cause everything from misfires to random dying, to starting and dying soon after start.