I have a 1993 Plymouth Voyager, 120,000 miles, 6cyl, that is only getting 9 mpg. When I first relaced the plugs and wiring, and air filter, ( 2 weeks ago), my mpg jumped to 18 mpg and now it has dropped down to 9 mpg. Could this be a faulty sensor robbing me of my mpg? I have checked for leaks, (none), and there is no smell of gasoline when driving or parked. Please help. Thanks.
Very well could be, seems that Oxygen sensors are more apt to do this. With a proper scanner, the oxygen wave patterns can be observed to see if they are lazy. Other problems that can affect gas miliage is a stuck thermostat. Does the van come up to temp and give good heat within 5-10 minutes?