big cam cummins

if it lost an injector tip it'd blow white smoke really bad, you haven't mentioned that so I'd rule that out, if it were white smoking then that cylinder would be extremely hotter than the others verses a misfiring injector which is gonna cause the cylinder to be cold. Scoring a piston is will cause a knock, broken injector tip will score a piston but now were back to white smoke. Scoring a piston can also be caused by a broken piston cooling nozzle but then you'd see a drop in oil pressure from the nozzle being broke. Running hot can score pistons but you've said all has been normal so I'm ruling out piston issues for now. I'd start with checking exhaust port temps, infrared temp gun works best, if not available then the crayon as stated earlier or even a squirt bottle w/ water sprayed in the ports to see which is coolest can help narrow it down. That's if it's an injector issue. Could be other things but that's a good starting place.

 

I still say injector, I have lost injectors on a big cam III without blowing the tip and therefore no excess smoke just a terrible knock. I also didnt get a huge temp difference btw cylinders using temp gun due to the fact that the inner part of the manifold is gonna be hotter due to design.

 

injectors are jus one of the easy things to check IMO. If u can't determine which one it is (if it is an injector) by the exhaust port temps, next I'd cut-out each individual injector until I found the bad one or ruled out the injectors. These older 855 engines you'll have to cut em out manually with a wrench. The Jakes or C-Brakes gotta come off and the 3 oil supply ports in the rocker boxes have to be plugged unless you make block-off plates. Question: is this an NTC or STC engine?

 

You don't want to start it with the jake brake off unless you want a oil shower. I would start it cold and keep feeling the exhaust ports to see which one doesn't warm up to find the miss. I was thinking about it, if its not an injector maybe there is a broken cam follower. If you get a long screwdriver or broom handle and stick one end up against your ear, use the other end to listen around on the block and try to isolate the knock.

 

when u remove the jakes, there's a Allen head screw in the rocker box that keeps the rocker shaft from moving, it's hollow and supplies oil to the jakes, u need to block that off or temporarily replace em with solid screws to block the oil. Once you've done that, take a 3/4 wrench and lift the injector rocker towards the exhaust side, this takes the cam accuation of the injector out of the equation and disables the injector. That's how u wanna go about isolating which injector changes the sound of the engine.

 

were u able to diagnose it?