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  1. #1
    Join Date
    Jan 2018
    Location
    Minnesota
    Posts
    2

    Question 2008 lmm overheat under load

    2008 LMM (240k miles) has developed an overheat issue.

    Can pull a 15,000# trailer easily without issues on flat roads and moderate temps without a problem. Temp gauge stays solid at 210°.


    On long modest grades, or steep short grades, it heats up to 260° fairly soon, but cools down quickly on next downgrade.


    Never heard the fan kick in, so changed out the fan clutch. Still don’t hear it.


    DEXCOOL always kept clean and regularly flushed.


    One hint — when it reaches 260°, just when I get a hot engine warning on DIC, it will SUDDENLY drop temp from 260 to 235 or so, then slower drop back down to normal.


    I’m wondering if the second thermostat is faulty and opening late, thus “keeping” hot coolant in the block?


    Comments?

  2. #2
    Join Date
    Dec 2002
    Location
    Teton Valley, Idaho
    Posts
    1,873

    Default

    Pull the thermostats and drop 'em in a pot of water, then bring that water to a boil. Did the thermostats open?

    Another check, especially with that many miles, would be to make sure the cooling stack is clean. There's been cases on this forum where birds have set up shop between the turbo intercooler and the radiator, effectively blocking air flow through the radiator.

  3. #3
    Join Date
    Apr 2001
    Location
    CA
    Posts
    13,573

    Arrow

    That hot with no fan clutch engagement (old, then new clutch), indicates hot air isn't reaching the clutch. The clutch is thermostat controlled. Hot coolant opens the thermostat(s), which allows the hot coolant to heat the air passing to the clutch. As said previous, either failed thermostat(s), or blocked airflow. It's really that simple.
    1985 Blazer 6.2
    2001 GMC 2500HD D/A
    dmaxmaverick@thedieselpage.com

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