DD15 coolant temp issue

Discussion in 'Heavy Duty Diesel Truck Mechanics Forum' started by Oleg D, Apr 19, 2026 at 8:56 AM.

  1. Apr 19, 2026 at 8:56 AM #1
    Oleg D

    Oleg D Bobtail Member

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    “Hello everyone, I’m looking for a coolant flow diagram for a Detroit Diesel DD15. We’re having an issue with a Freightliner Cascadia where the engine cools down quickly when not under load. While driving downhill, the temperature drops to about 150°F, and at idle it won’t go above that. The thermostat have already been checked and replaced. Any advice or suggestions would be greatly appreciated.”
     
    Last edited: Apr 19, 2026 at 9:34 AM
    Oleg D, Apr 19, 2026 at 8:56 AM
    Oleg D, Apr 19, 2026 at 8:56 AM
    #1
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  3. Apr 19, 2026 at 10:35 AM #2
    JB7

    JB7 Road Train Member

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    Drops to 150 according to what, the gauge on the dash? What is the ECM reading for engine temperature. What is your coolant protection at? Too much or too little antifreeze (which also raises the boiling point) can cause problems. What is the fan clutch doing, is it on all the time?
     
    JB7, Apr 19, 2026 at 10:35 AM
    JB7, Apr 19, 2026 at 10:35 AM
    #2
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