I have a 300sd (83) and trying to combat a knocking issue. I have read on here to check your glow plugs. Could someone explain what that has to do with it. I thought the glow plugs "heated up" for ignition? Then turn off after the engine has started. If one plug was bad would it cause that cyclinder to never ignite (that may be a stupid question)?

I pulled the plug out of the cyclinder that seems to be doing the knocking (opened the fuel line and it stopped), which switching the injector didn't seem to fix and after I replaced the glow plug the knocking stopped.....for a little while. The plug was pretty hard to get out....there was a ton of black gunk around it. I later read that there is a "reamer" tool used to clean the opening. If the opening is all gunked up....does it cause that plug to faulter?......And, then again it leads me back to my original question....even if that plug is bad....if the engine started....wouldn't that cylinder still fire up? Why does that have anything to do with knocking?

rshafferdc