hole in the exhaust messes up the fuel economy,same as like the hole in the govament
sensors are under the distributer,the dash temp gauge(one wire) and the coolant temp one(2 wire i think)
oxygen sensor is on the front of the manifold
just going by what frank said about the thermostat may be stuck open,worth a check
and the sensors have a certain resistance (ohms ) at certain temperature
you could get a cheap multimeter at maplins or else where for around a fiver , then check em out