DD15 coolant temp issue

Discussion in 'Heavy Duty Diesel Truck Mechanics Forum' started by Oleg D, Apr 19, 2026.

  1. Apr 19, 2026 #1
    Oleg D

    Oleg D Bobtail Member

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    Jun 29, 2021
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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
    Oleg D, Apr 19, 2026
    Oleg D, Apr 19, 2026
    #1
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  3. Apr 19, 2026 #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
    JB7, Apr 19, 2026
    #2
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  4. Apr 20, 2026 #3
    Magoo1968

    Magoo1968 Road Train Member

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    St Malo mb
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    Fan is on or bad faulty replacement thermostat on older models will do that on newer models it could be a problem with the variable speed water pump. We convert out variable speed water pumps to old style and reprogram computer when they need replacement instead of paying a grand every time for a inferior product in the mountains.⛰️
     
    Magoo1968, Apr 20, 2026
    Magoo1968, Apr 20, 2026
    #3
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  5. Apr 27, 2026 at 12:17 AM #4
    khurmiDiesel

    khurmiDiesel Bobtail Member

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    Apr 5, 2026
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    Water pump could be faulty
     
    khurmiDiesel, Apr 27, 2026 at 12:17 AM
    khurmiDiesel, Apr 27, 2026 at 12:17 AM
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    Rideandrepair Thanks this.
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