I started the 240 and immediately heard a loud pop, like a backfire. Mechanic had cross threaded a new oxygen sensor, it came loose, and the car backfired and immediately died. Restarted and had trouble maintaining idle but ran ok as long as revs were up. Took foot off gas pedal and car died again. Wouldn't maintain idle. Replaced timing belt, checked all vacuum hoses, distributor and coil wires, spark plug wires, even took off the catalytic converter thinking it might have been stopped up. Nothing helps. Something happened when the thing backfired and can't figure out what it is. This is a B21 engine with K-Jetronic and the constant idle speed system. Please help.