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Old 04-27-2019, 08:55 PM
hansbrakob hansbrakob is offline
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Location: Minnesota
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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?


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Old 04-27-2019, 08:59 PM
rapidoxidationman rapidoxidationman is online now
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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.
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Old 04-27-2019, 11:04 PM
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DmaxMaverick DmaxMaverick is offline
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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.
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