LLY Bad Fuel Knock.

[Redbowties88]

Hey everyone, I'm trying to help out a friend of a friend with his newly acquired LLY dually.

You start the truck and it has a nasty knock from the passenger side around numbers 7&5. After looking under the hood I can tell he definitely has some wiring issues where chipmunks or squirrels have been in there tearing things up.

Anyway I hooked up the scan tool and using the DVT I started shutting down injectors and when I got to #7 Wula it stopped knocking. So I told the guy about the ice pick trick and the harness problem that it would most likely need sooner or later anyway so start with that and if that still doesn't work you might need an injector.

I got a call today that after changing the harness and the injector it's still knocking just as bad. I'm about stumped. to make things more confusing they ran the truck with #7 injector harness disconnected and it still ran horrible and was knocking. They hooked #7 back up and unhooked #5 and he said at that point it ran much better

Rail pressure is normal and nor erratic.
Balance rates are all at 0.
Codes include, Underboost, FPR voltage, #7 injector circuit open, and a few other but my friend doesn't remember them all. But there was one more regarding injector voltage


Any ideas for this?

 

[messejme]

I believe the assignment of injector numbers in EFILive is not accurate. the actual cylinder # displayed in EFILive is mismatched.

 

[DAVe3283]

I'd write down all the codes, clear them, and see which ones come back. That should let you know where to get started.

And for a while, on the LLY, the cylinder # displayed in the DVT was wrong. I heard they fixed it, but I also heard it is not fixed. Either way, don't trust what the DVT tab says.

However, the codes themselves come from GM, and can be trusted. If there is an injector #7 open circuit code, then it is in the injector 7 wiring. But that could have been from when your friend was diagnosing it, hence why you should clear them and see what comes back.

 

[Redbowties88]

the truck has definite wiring problems... its been chewed up really badly and poorly repaired.

But the wiring from #7 to the ficm appears to be in tact.