hello im hoping you people can help i recently bought a ford ka 1.3 found the car wasnt picking up speed like it should so i got the oil and spark plugs changed cars seems ok now just slows down a little when going up a hill now my engine has started rumberling while in 1st and 2nd gear sounds fine in 3rd ive had someone look at it and they have said plug 4 is misfiring so i have to pay for new plug leads. im just wondering if this will deffinatly stop the rumberling or could it be anything else??

thank you