Why do I get very Bad gas mileage in 1994 Volvo 940 wagon?

If a complete tune-up (with new ignition wires, plugs, air filter, cap, rotor) does not improve gas mileage, replace the coolant temperature sensor. The sensor is located under the intake manifold. Cost is about $39.00. Check the shape of the wiring socket to make sure you get the right sensor.