I got a 89 f250 with a 300 4by4 5 speed standard. i got a bad knocking noise coming out from the back of engine. I pulled the plugs and the # 6 plug has some oil on it. if im driving slow around town it sounds like crap and doesnt run good, but when i drive faster the knocking somewhat goes away. it starts right up with no hesitation. could it be a bad injector?? any help appreciated