I have a 1993 F450 7.3 IDI with a knock I'm trying to diagnose. If I crack the fuel line to the inner passenger side cylinder it goes away. Once you tighten it back up it come back loud and clear. The engine runs smooth normally but when I crack the fuel line the injector stops working and the engine clearly begins to shake. All the searching online has me guessing between injector and bearing/wrist pin. I've never done this work before so I'd have to pay for this but don't want to just start guessing at a fix, instead of wasting money on tshooting a possible injector i'd rather put the money to a replacement motor. It stands to reason that if the cylinder is not firing then it's not under pressure so the bearing/wrist pin is quite but at the same time I would think that if it's indeed mechanical as I rev it up with the fuel line cracked open I would still hear a knock just from high RPMs on those parts. Any ideas or suggestions would be greatly appreciated!!

 

You can get one injector for $40 and change it out with a cresent wrench. That would be the cheapest way forward. If that doesn't fix it then you can start looking for a replacement engine

 

i agree. swap in a different injector.

 

More than likely injector.