do you know for sure that you have spark plugs. my 99 ford expedition has individual ignition coils in the same place the plugs would go. they are about 60 dollar a piece. when one goes the car will run bad. you should be able to take it up to autozone and have check it eith the dianostic tool for free. it might give a code especially if your check engine light is on. either way im getting rid of expedtion as soon as i get the chance. it has been alot of problems.