hi, i have a problem with my ford escort. when the engine is idlling its spluttery and jittery i came o the conclusion that its running on 3 cylinders. i removed the spark plug leads one at a time and the one on cylinder 4 is the only one that doesnt seem to have any effect on the engine when its removed (not firiing). but there is a spark coming from the lead.

im hoping that a new set of spark plugs will do the trick but noticed someone elses post on here from a few years ago that someone had the same problem and had to have their injectors cleaned and all sorts but still didnt solve the problem after changing the plugs and leads aswell. anyone know what else could be wrong or any tips incase changing the spark plugs doesnt work?

thanks in advance