Usually bad gas mileage is related to Oxygen Sensors going bad or a leak in the exhaust near them. The fuel pump prime is hard to hear. You may need a second person to get near the tank and listen for it.
The other issue may be the idle air control valve which keeps the idle steady by opening ports to change the mixture. If it is stuck in a rich air/fuel ratio setting, that may be why the car gets bad gas mileage. It may need to be cleaned or replaced.
You may also have a weak coil. Test the coil and see if it is bad.

Answered by DrCranknWrench (expert)
3,292 answers provided

An ASD relay is something used on Chrysler vehicles, not GM. You really need a good scan tool here to be able to see all the sensor data to look for something out of place. You could see if the computer is trying to compensate for a rich condition or if one of the temp sensors is not reporting accurately.

Answered by Wrenchtech (expert)
14,716 answers provided