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

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