The manifold being cracked will REALLY lead to alot of problems(see above), since you basically have a place for fresh air in enter the exhaust stream ahead of the O2 sensor, the computer is tricked into thinking that there is a lean condition in the motor, and tries to correct it by adding fuel to fatten up the mixture, which leads to bad economy, and a possibly "smoked" converter from the over-rich condtion. The O2 sensor will also tend to have a shorter life because of the excess fuel "coking" or sooting up the element inside it. Let's also not forget that this cracked manifold can be a POTENTIAL place for a fire to start, since oil, grease, and fuel lines are close to this area.