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  #1  
Old 12-27-2007, 10:10 PM
scooper scooper is offline
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Join Date: Dec 2007
Location: Regina, Saskatchewan, Canada
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Angry 2007 GMC Medium Duty with 7.8L Isuzu Engine

Hello, I am new to this forum and also new to the GM Truck.
Bought band new in October 07, have had nothing but grief with this truck.
Can someone please help:

Truck has had loss of power, bucking, jerking. Taken it to 6 different GM dealers, no codes show up at all. Yesterday check engine light came on, drove back 180 miles to dealership, started truck in morning light was still on, got to dealership shut truck off, drove in garage, no light on, no codes nothing.

Loss of power, bucking, jerking, upon acceleration?????
When truck goes into reduced power mode, we accelerate hard to bring speed up, then the regneration light comes on to clean DPF filter.

Why are there no codes showing???? Dealer says nothing wrong, no codes, don't know what to fix.
Anyone please help!!!!!!!!
Sensors??????? which ones?????
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  #2  
Old 12-28-2007, 04:56 AM
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Mark Rinker Mark Rinker is offline
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Welcome aboard. Sorry to hear of your troubles - especially with a brand new truck! I own a C4500 Kodiak - unfortunately, the number of medium duty owners here on TheDieselPage.com currently is low. I don't recall seeing any posts from other 7.8L owners. However, some of the problems you may be encountering could be common across the Duramax family, the new DPF systems, or diesels in general.

Usually, 'bucking' is fuel related. Something is interrupting fuel flow or varying fuel pressures. Being it is winter - could you be experiencing gelling problems? What fuel are you running? Have you tried changing the fuel filter(s) and running a winter blend fuel or #1 diesel?

Fill out your profile and signature line with location and truck info - it will help us to troubleshoot - i.e. if you live in Cali, please disregard the gelling suggestion...

(Moderator, please move this thread to the Duramax forum - we'll move it later to the Medium Duty forum.)
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2005 GMC Yukon Denali 6.0L gas (hey its paid for)
  • Previous owner of two 1994 6.5L K3500s, '01, '02, and '05 6.6L K2500s, '04 C4500, '06 K3500 dually, '06 K3500 SRW, '09 K3500HD SRW
  • Total GM diesel miles to date : ~950K
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  #3  
Old 12-28-2007, 06:59 AM
scooper scooper is offline
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Location: Regina, Saskatchewan, Canada
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Default 2007 GMC Medium Duty

Sorry I don't know how to fill out profile, but we live Regina, Sask, Canada.
We have to use ultra low sufur fuel @ 15ppm, and low ash oil, which we are doing. We run all over North America, fuel filters have been changed at 6 different dealerships in the U.S. and Canada still have the same problem.
It shows no codes, and until a code shows they tell us they cannot fix.

The truck will start off good, then at any time it will go into the low power mode, no codes, nothing.

Fuel pressure was tested, this showed no leaks.

We pull a 51 foot beaver tail and can't even make the hills in Minnesota/ Wisconsin.
With no codes showing up dealership said they can't repair.
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  #4  
Old 12-28-2007, 07:59 AM
mr. monte mr. monte is offline
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we have the "Lemon Law" here in the States

maybe something for you to check out
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  #5  
Old 12-28-2007, 10:37 AM
DmaxMaverick DmaxMaverick is offline
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Location: CA
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Welcome to the Forums!

To get your signature filled out, go to your User CP options, and select signature. Basic info is very helpful (use other's as an example), but your truck isn't as common around here, so more detail is better. If you can't figure it out, please email or PM (Private Message) me or a moderator, like Mark. I'll move the thread to the Medium Duty Forum. The Duramax Forum is intended for the 6.6L Duramax.

Now, as far as your problem goes, it doesn't sound really unique to the 7.8L. Most problems are Diesel engine related, and can apply to any Diesel engine. The Dealer can diagnose problems w/o codes, but many are too lazy (for lack of a better term) to do real technician work anymore. You need to find a good Medium/Heavy Duty GM/GMC authorized dealership to take care of you. Very few issues that cause poor running will cause no codes. Fuel quality is one of them. If you are not using an anti-gel additive, try it. It could be simple as that. Also, check the air filter. If it's getting any moisture in it (like an airbox leak), it will freeze up and block airflow. If it were a sensor problem, you'd certainly have DTC's (SES lamp) stored in the PCM memory, which will still be there after the SES lamp goes out. And, it could be a transmission issue, which will cause low power mode and the SES lamp, but needs to be diagnosed separately from the engine.

We'd like to help you, and will given the chance. At the very least, someone else will be helped by the experience. Please keep us updated often with any new information.
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  #6  
Old 01-23-2008, 06:06 PM
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Duramaster Duramaster is offline
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I work on 7.8's occasionally. Now more often than not since our 7.8 guy is off on injury. I will ask him a question or two.
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  #7  
Old 02-04-2008, 11:36 PM
GregCrabb GregCrabb is offline
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I know this is a totally different engine made in a totally different country and does completely different work from yours....but....my Volkswagen diesel was doing the exact same thing your Truck is doing....and it was my ECU simply telling my turbo not to boost. The engine would start fine, idle fine, drive fine until around 25 mph, then it was as though the bottom fell out and it would crawl. It took my dealership a month to finally figure it out, but after I got a new ECU under warranty, it has been fine since, knock on wood. Just a thought. Mine would always act up on my interstate drive to work in bumper to bumper traffic at 70 mph...and my car would be struggling to do 45. They first thought it was the EGR sticking, then the MAF sensor.

Like I said though, I know my engine is not like yours.
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  #8  
Old 02-06-2008, 07:53 PM
scooper scooper is offline
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Thanks for the info about your volkswagon, but the engineers changed all the sensors, 6 injectors, two ECM's, two emission pipes, still doing the same thing, that is when they decided to give us a new engine. Our old engine only has 20,000 miles on it. Bucking, jerking, loss of power, basically from day one, not as bad, but got progressively worse. First oil change at 5000 miles, oil real black, secondary fuel filter really black in the vanes of the paper filter.
Alot of carbon, I would think.
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  #9  
Old 02-10-2008, 06:33 PM
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Mark Rinker Mark Rinker is offline
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Fuel filter really black? Ummm...where are you getting your fuel??? As I said in the first post - bucking/jerking sure sounds fuel related. Now you report your secondary fuel filter is black? A lot of carbon? In your fuel?

Better get that one figured out - before you feed a brand new engine the same fuel that killed the first one.
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2005 GMC Yukon Denali 6.0L gas (hey its paid for)
  • Previous owner of two 1994 6.5L K3500s, '01, '02, and '05 6.6L K2500s, '04 C4500, '06 K3500 dually, '06 K3500 SRW, '09 K3500HD SRW
  • Total GM diesel miles to date : ~950K
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  #10  
Old 02-13-2008, 11:54 PM
murphyslaw murphyslaw is offline
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sorry to hear of your problems.

I have an '03 6500, it has been a great truck.

It did go into limp once. I was unloaded and going up a hill that I went up everyday and just dropped to idle and the SES light came on. I pulled over shut it off for about 2min started it back up and have put about 30k on it since. and this happened at 88k.

if your fuel filter is black you have fuel quality problems.
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donor rig. 1988 GMC K3500 140k miles, th400. IFS(read CRAP) semi float 14 rear. drove it 142 miles home at 17mpg and 1/4 quart of delo 400.
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