I have a 1994 ford ranger splash and it needs help!?

i have a 1994 ford ranger splash. it has a slight miss in it when my mass air flow sensor is hooked up. when i unplug the sensor it stops the missing. either way if i have the air flow sensor plugged up or not it still get really bad gas millage compared to what it so post to get . it gets 12 mpg now and it use to get 19 to 20 mpg.
i have replaced the mass air flow sensor and reset the computer by unhooking the battery terminals. it will run good with the new sensor on for about 5 minutes. then the missing starts again.
can any one give me any suggestions?

Just a stab in the dark here. sounds like a vacum leak. Have a mec. look at it.