http://www.dieselplace.com/forum/smileys/Big Smile.gif


I have a stock 02 3500 Duelly that I tow a 37' fifthwheel that weighs in at about 14000# When I'm towing up any kind of grade and the engine is turing 2400 - 2600 RPM there is a noticeable knock in the engine. If I back off the throttle the knock goes away. I had it in the shop for this problem and the mechanic could not duplicate the problem and he found no codes on his scanner even while road testing the truck. I still have plenty of pulling power so performance is not an issue. I just don't want the engine to come apart. Has anybody had a similar experience and or solution to the problem. And no the injectors have not been changed.


Thanks,


Brad

Don't worry!


This is a very common complaint with new owners. The pilot injection is the technology that makes the newer diesels more quiet than the older ones. It can only work up to a certain RPM right around 2600. After that the injectors have to change the way they work in order to remain efficent. When the pilot injection turns off the engine becomes much louder. When you let off the RPM's drop and the pilot injection turns back on. As long as there is no power loss your fine.

Take the dealer for a ride under load and going up hill.
Demand that the certified diesel mech. hears the noise and logs it.

I agree about the pilot injection. When I first heard it, it threw me for a loop.

Gents


The pilot injection explaination sure makes sense, but where can I find additional information on the subject.





Brad

The behavior of the pilot injection is described in a TSB that is related to overdrive lockout (because when it is hunting for a gear it alternatively can go above and below the rpm and fuel flow where the pilot injection is on and off). The TSB can be found at :


http://dieselplace.com/forum/forum_posts.asp?TID=753&PN=1


In the TSB referenced above, it states :


"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 notice able. 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."


Note that the pilot injection shutting down is dependent on both rpm and fuel supplied to the injector.


jeffEdited by: dmax lover

Mine does it without a load, so yours probably does too.

I noticed the knock at about 1100 miles. It happens only when under load. If I let up slightly on the gas it goes away. Most noticeable when on the freeway at about 65. Definitely way before 2500/2600 rpm. I'll hear it easily at 2000 rpm and below.


I took it to the dealer. After the first day they thought it was a bad injector. After the second day they said it was throwing two codes, one for the turbo and one for the egr but the service manager did not have the code numbers. This is the third day, they just called me and told me the 'EGR control module' wasn't working and needed to be replaced. They told me they were so new they would have to pull one off the line and it would probably take about a week.


They gave me a Envoy as a rental the first day so I'm o.k. with it taking time as long as they fix it right. Also don't want to drive it if that would hurt it. I know knocks are not as bad in diesels ad gassers but it still makes me cringe when I hear it.


I'm new to diesels and the duramax but I've been reading the board heavily for the last couple of months so I've picked up some stuff. This sounds kind of fishy and I asked them why the egr module not working would cause a knock an they just said its possible. Anyone have any ideas?


It runs fine. No black or white smoke. Gets good mileage - 18-20mpg on the highway at 65mph. The coolant temp goes up to a couple lines below 210 and sits there when fully warm on a hot day. Not sure if this good or bad for this engine but I haven't towed anything heavy to see if it goes higher.


Sorry for the long post. Included as much info as possible to try to anticipate any questions.

dmaxfan - is that a Basset Hound I see?

dmaxfan - is that a Basset Hound I see?


Yep, that's Fred go to the off topic forum and look for pictures of my boy.

dmaxfan,





I just returned from a week of camping with my fifyhwheel. The perceived problem I thought I was having was hit right on the head by the earlier responses. (Thanks Guys) Right at 2600 it starts to knock. I also installed a K&N intake and a 4" pinnacle exhaust system to solve an over exhaust temp problem I was experienceing. The engine is definitely stronger pulling and the mileage was up to a little over 10MPG (14K lb RV) too boot, and that was pulling over hill and dale. When I had my diesel checked out at the dealer awhile back there were no codes indicated. I suspect that you may have one of the newer diesels (LLY series) that may have a different set of circumstances to deal with. Good luck with your EGR module. I suspect that the mechanics are learning about new the new diesel along with you. Not uncommon now days.





Brad

Brad,


Glad we were able to help out. Keep the posts coming.http://www.dieselplace.com/forum/smileys/Big Smile.gif

My'04 LB7 shuts the piolt injection off at 2000rpm's when on the gas. I don't know what rpm it does it when towing up a hill. Just my opersavtion.

It sounds to me like you need to take your truck to a competent GM diesel mechanic and have it checked out (warranty). I'm guessing that something isn't working right, but that is up to them to determine. Talk to the mechanic and make sure be knows exactly what your complaint is. Refer him to the above TSB provided from "DMAX LOVER" if necessary.





~B

Shutting off at 2600 RPM is not written in concrete. Mine has shut off at 2400 since day one. No way could I check it while on the go peddle. It accelerates so had and my old ears don't work so good anymore. I have noticed a lot of times when towing that it stays off below 2200. Down hill in tow/haul load braking is one place it is very loud after it shifts back up. Later! Frank