Engine light

I have a 91 ranger 4.0liter with a manual transmission. The check engine light comes on as I'm driving in town for more than a few minutes. when im on the highway it usually doesn't come on until I go through a town. My fuel economy is very poor 14-16 mpg highway. I have changed the o2 sensor ,sparkplugs,and air filter . It runs somewhat better now but the light still comes on and the fuel economy is still poor. the idle is a lil rough it goes up and down some but its not as bad since i changed the plugs. anyone know what this might be?

Do you have a K&N filter?

I have a 91 ranger 4.0liter with a manual transmission. The check engine light comes on as I'm driving in town for more than a few minutes. when im on the highway it usually doesn't come on until I go through a town. My fuel economy is very poor 14-16 mpg highway. I have changed the o2 sensor ,sparkplugs,and air filter . It runs somewhat better now but the light still comes on and the fuel economy is still poor. the idle is a lil rough it goes up and down some but its not as bad since i changed the plugs. anyone know what this might be?

There's so many factors that could cause the issue. You best bet would probably be to run the codes and see if the computer sees any problems. Since it's a 91', it's possible to blink out the codes on a voltmeter or CEL with a paper clip.

Pete