What's this noise, not the idle tapping

[markm]

I pull a 10,000 lb fifth wheel trailer with my 2500 HD Duramax/Allison. When accelerating hard up a hill, it will make a loud metallic tapping noise over 2500-2600 rpms. What causes this noise?

[TooMuchMuscle]

sounds like the pilot injectors are cutting off...

[dmaxalliTech]

Sounds like your loosing pilot injection. The question is, Why? Most likely you have an injector headed south. The last few that I have had with similer noise, I could watch balancing rates on scanner and would have one above 12mm, spec is -4 to 4mm. As noted before, you cant trust that info to condemn injector, but I would get it in to have it further checked out

[TooMuchMuscle]

I get the same tapping noise when I hit 85mph on the highway... I thought it was normal for the pilot injectors to turn of at that high sustained mph/rpm... Looks like i am heading for the dealer...

[conradv]

Now I'm confused - I searched for "Pilot Injector" and read about 20 posts.

Mine cut out at about 2600 rpm and make the same noise that I read on the previous posts - Yep, it sounds like an old Cummins... ;)

So are these "tapping" noises different than just the normal sound of the Pilot Injectors shutting down due to high rpms? That would be the only reason to take it to a dealer...

[Modified]

Pilot injection is being disabled, as designed. Here's some info from GM,that I received with their ODLO installation info:

Diesel engine fuel injection systems that use a common rail system, as the Duramax diesel does, may or may not have what is referred to as pilot fuel injection at the lower engine speeds. Pilot fuel injection is defined as a pre-injection of a small quantity of fuel 5 to 20 crankshaft degrees before the main fuel injection. This pre-injection softens the main injection pressure rise and reduces combustion noise. At engine speeds above 2600 RPM, and a fuel quantity supply greater than 35 cu mm, it is necessary to eliminate pilot injection to meet emission requirements. When driving at or above these speeds, main injection combustion noise may be noticeable. It is normal and not harmful to the engine.
When traveling in 5th gear, the pilot injection to main injection transition is typically in the 136km/h (85 mph) range, however it may also be heard under heavy acceleration and when pulling loads up a hill during shifts at or above 2600 RPM. Customers that install the fifth gear inhibit switch, (ODLO), may encounter the fuel knocking of main fuel injection at speeds around 107 km/h (67 mph) in fourth gear between 2600 and 2700 RPM. This sound is considered normal and is to be expected.

[conradv]

Modified - that's what I thought.

The first time I heard it, I hit a hill and the rpm's went up - startled me a bit, because I was so used to the pilot injection quieting the engine - thanks to this site's search engine, I was able to figure it out in no time. I was curious if there was another tapping noise to be listening for (due to NUMEROUS fuel filter / injector failure discussions...)

Thanks for the info.

[ 03-20-2003: Message edited by: conradv ]</p>

[Oongawah]

I haven't noticed it on my truck. Maybe the juice makes a difference in this area.

[Kennedy]

This occurrence is definitely reduced/masked with the addition of a power module.

The ODLO does DEFINITELY make a difference in engine tone as desribed above...