Results 1 to 16 of 16

Thread: 2007 GMC Medium Duty with 7.8L Isuzu Engine

Hybrid View

Previous Post Previous Post   Next Post Next Post
  1. #1
    Join Date
    Dec 2007
    Regina, Saskatchewan, Canada

    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?????

  2. #2


    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 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.)
    2011 Chevrolet Tahoe 5.3L daily driver
    • 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, '05 Denali
    • Total GM diesel miles to date : ~950K

  3. #3
    Join Date
    Dec 2007
    Regina, Saskatchewan, Canada

    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.

  4. #4
    Join Date
    Jul 2007
    ortonville, mi


    we have the "Lemon Law" here in the States

    maybe something for you to check out
    07 GMC CC SB LBZ
    70 monte carlo, original 4 spd, bench seat
    71 monte carlo SS clone, street/strip
    97 grand am, beater

  5. #5
    Join Date
    Apr 2001


    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.
    1985 Blazer 6.2
    2001 GMC 2500HD D/A

  6. #6
    Join Date
    Mar 2007
    Pacific Northwest

    Thumbs up

    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.
    1998 Chevy k-2500 454 147,000 miles and climbing!

    1993 Pontiac G/A Big Block 3.3 Liter 140,000 miles and still going too!

  7. #7
    Join Date
    Feb 2013
    hannibal MO

    Default greg

    I have a 2007 T6500 GMC and I cant get the engine to stop powering down because of the emissions bull the check engine lite codes all indicate the emissions can anyone help.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts