Simple question. What is injector knock and what causes it?

 

Come on, nobody can help?

 

Injector knock or fuel knock is the rapid or uneven burnig of fuel caused by one or more injectors that are not perfectly spot on. It typically is only heard at idle or light throttle applications. During light throttle the timing is advanced and that can increase the sound. The sound is a lot like the diesel knock itself but more intermittant and not as dominant.

It is the result of the injector not being able to atomize and spray the infitesimal amount of fuel injected at idle and light load. The fuel spray pattern may not be exact or there may be dribbling of fuel at the cutoff when the injector closes or one of the nozzle holes may be partially plugged.

It can be a tricky thing to diagnose, even with good diagnostic software and even by individually bench testing injectors. If it is doing it at idle you might listen to each injector fuel line with a stethescope or screwdriver to your ear to figure out which is loudest. Even then not always distinctive.

In most cases when it is just a little ticking and not present at the medium or higher loads there is no issue with it and no loss of economy. If it persists at higher loads and egts are above normal then it can burn a hole in the piston. But that is not likely without other symptoms like vibration or smoke or egts way out of normal range or too much timing advance.

If you are running a fuel programmer and it is advancing the timing you may want to set it to stock to see if the knock goes away.

Hope this has helped

 

I just replaced all 6 injectors in March. Whats the chance of any of them going bad that quick? I was driving at 70mph when it had 2 hard misses then lost power. I stomped it and blew thick black smoke, much more than normal. When I got slowed down so I could hear is when I heard the knock. Limped it home(about 10 miles) and listened close when I got home and it sounded like a rod knock, only it was in the top end. I called a guy that builds high performance diesels and he said it was injector knock but didn't enlighten me as to what caused it or what I have to do to fix it. He said bring the truck and 400 dollars and he would do a leak-down on it and tell me exactly what is wrong with it. 400 for a diagnostic that I don't have. Couldn't I pull the injectors and have them checked somewhere?

 

I cannot speak for our common rail Cummins as my experience there is limited. I managed a CAT truck engine shop in the late 90's through 2003 and do have extensive electronic fuel system experience. We changed a lot of electronic injectors, and I mean a lot.

Basically, to answer your question, Yes, it is not unheard of that new injector go bad quickly. There are a number of reasons but here is what is most common.

1. The new injector passed bench tests but was defective, had dirt in it, etc. and fails in short hours. Bad from the factory. The longer it goes after it is installed the better the chances it will keep going.

2. The original cause of injector failure has not been addressed in your truck and new injectors are suffering the same consequence. (If that is why you replaced them in the first place. If not disregard this line item) Dirty fuel, air in the fuel, water in the fuel, low fuel pressure, high fuel pressure, system problem.

3. The installer did something wrong or contamination got in the system when it was opened up for the injector change.

You should have parts warranty on the bad injectors, if that is the problem. You should talk to the original installer and parts supplier of the injectors to file a claim. Even if you are not near them now they may compensate you to have things checked out if you take it someplace they authorize.

I believe the injectors can be bench tested as can much of the components in the fuel system such as the high pressure rail pump and electronics.
I;m not sure what they are calling a leak down test. That term is used to measure engine cylinder sealing for rings and valves. However they may be referring to injector leakage like dribbling or dripping as mentioned in my first post. I would focus on the injectors and fuel system and don't rule out the electronics and wiring harness either.

Why was the injector change done? Who did it? Who supplied the new injectors? How many miles on the truck?

One more thing. If the injector tip is blown off you need to pull the exhaust housing off the turbo as it is likely the turbo exhaust wheel is damaged. A common problem with blown injector tip. At CAT if our new injector was bad and ruined the turbo we warranteed the injector, labor, and turbo. That was CAT factory parts warranty. At least when I was there.

Others in the fuel system repair business need to chime in.

 

Did you install the injectors yourself? Give us the particulars on your truck.

 

The truck is an 04.5 3500 4x4 extended cab long bed dually,5.9 325hp. A guy I know that does his work on his own Dodge put the injectors in. He's done it before so I trust we did it right. It's been running fine until the other day. I replaced them because it had an intermittent miss and was told it was probably due to bad injectors. It sounded like it had a dead cylinder at idle and you could feel the miss occasionally while driving. Truck had 117,000 miles on it when I bought it and only has 127,000 now. It has quite a bit of blowby from the crankcase but I can live with that for now. I just need to fix this problem I've now without having to sink another 2 grand in injectors. I put in new Bosch stock replacement injectors. I've been working on cars and race cars for a long time but this diesel crap is all new to me. I'll take all the help I can get.
Thanks.

 

Oh yea, the truck shows no codes of any kind.