I think I my have a electrical problem on my 03 GMC DMAX. As soon as I start the motor the voltage gauge reads about 10 to 11 volts for about 15 to 20 seconds, then the gauge will go up to about 14 volts. While this is happening the dash lights are dim and the windsheild wippers are noticably slower. I just replaced the batteries and it was doing the same before they were replaced.
I also noticed when I am traveling on the interstate and I speed up to pass at about 85 mph I hear a noticable differance in the sound of the engine. It sounds more ratley, is this the waste gate opening? Thanks.

JKeaser
The low voltage at start-up is the norm for these trucks. The louder rattling you here is the pilot injection turning off at a certian RPM I'm not sure of the RPM but that is normal as well. If someone else wants to chime in feel free :)

I think I my have a electrical problem on my 03 GMC DMAX. As soon as I start the motor the voltage gauge reads about 10 to 11 volts for about 15 to 20 seconds, then the gauge will go up to about 14 volts. While this is happening the dash lights are dim and the windsheild wippers are noticably slower. I just replaced the batteries and it was doing the same before they were replaced.
I also noticed when I am traveling on the interstate and I speed up to pass at about 85 mph I hear a noticable differance in the sound of the engine. It sounds more ratley, is this the waste gate opening? Thanks.

if the start-up readings you mention come when you first crank it after it has been sitting long enough to activate the "wait to start light," the meter is probably reflecting the current draw from the glow plugs and battery drain associated with cranking a diesel ... this is normal .... the ratley sound sound could well be the pilot injection or waste gate but that can't be said for sure without having someone who knows what they are doing to listen to it

Jim

What MICKD said. The low voltage is the heater element in the intake. The louder "diesel sound" is your pilot injection turning off at a high enough RPM - I think 2800+

Mine does the exact same thing. Definitely the heater element on startup and definitely the pilot injection shutting down at high engine rpm. No worries.

I know this is stuppid quiestion but what is pilot injection turning off and what is it for.... and I know that one day was -35c and my plugin braker went out I had to start the truck with out being plug in it took the volt meater to pass 12v almost 5 min...I suspected the glow plugs were still cycling after the engine sounded running on 5 cyl...extremely rough....but I could be wrong...

The duramax has an intake heater and glow plugs. There will be a voltage draw at startup every time, the length of the cycles changes with ambient and engine temperature. I use the voltage gauge as a "warmup" indicator and will wait until the volts recover before going into gear and moving out - gives a little more time for warmup and lubrication before loading the engine. Pilot injection is the reason for the quiet operation of the Bosch High Pressure Common Rail system. A small "pilot" shot of injection is shot into the cylinder to begin combustion prior to the main injection shot. This creates a smoother and more uniform burn wave in the main shot and prevents the classic diesel "chatter" which is actually a shock wave in the burning fuel in the cylinder. When rpms get too high (somewhere between 2500 and 2800 rpm), there's not enough time in the cycle to make a pilot shot - the injection happens in one shot, which results in the normal diesel clatter. I think you should take a few minutes to read your diesel supplement to the owner's manual - it will give you some very useful information.

thanks I may do that....

Pilot Injection:

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 136 km/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 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.


Reference: http://dieselplace.com/forum/showthread.php?t=708