I'm trying to find out the cause of my poor mpg on my 1992 300sd. I replaced the o-ring seals on top of the injector pump. My mpg was great for about a week, and then went back to poor mpg again. Could bad injectors cause poor mpg? I looked closely and don't see any leaks on the pump or around the injectors thanks. I don't know if it's related but my fuel cap is rusted out not sure how that happened.