The idle speed may be too low to allow the engine to idle. This can be caused by several things. You have to make sure the throttle settings are correct, the throttle body is clean, and if it has an idle valve, it is clean and functioning properly. If the vehicle has an idle switch or a throttle position sensor, it must be working properly as well. If all that is ok then you check for vacuum leaks, ignition problems, or fuel supply problems. A plugged exhaust could do it, but it sounds like you addressed that. Good Luck, let us know what you find out.