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Block Heater Replacement Detroit Diesel 671 TI

Discussion in 'Engines' started by Mickey Szypot, Feb 19, 2026.

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  1. Mickey Szypot, Feb 19, 2026
    #1
    Mickey Szypot

    Mickey Szypot New Member

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    I have a pair of 1985 J&T Detroit 671 TI's in my boat. I know how to remove and replace the block heaters. What I don't know is, how to empty the block prior to removing the block heater. My block heaters are located behind the fuel pump just above the stater motors.
    Can anyone give me some insight as to how to empty the block without the antifreeze going into the bilge?
    Thanks for your help.
    Mickey
    RT46 likes this.
  2. Capt Ralph, Feb 19, 2026
    #2
    Capt Ralph

    Capt Ralph Senior Member

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    If you have the screw in element heaters, you would be amazed how little coolant dribbles out when replacing the element.
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  3. Mickey Szypot, Feb 20, 2026
    #3
    Mickey Szypot

    Mickey Szypot New Member

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    That would be ideal, however the block heaters are held in with two bolts and a gasket in between. Thanks for your quick response.
  4. Capt Ralph, Feb 20, 2026
    #4
    Capt Ralph

    Capt Ralph Senior Member

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    Interesting.
    Then your install looks like this?
    s-l1600a.jpg

    Make sure it is really the element and not the power cord that is bad.
    Then on a cold engine;
    Do not open your heat exchanger cap.
    Have your new heater assembly ready.
    Remove your proven defective element assembly out. Stick a plug or rag in there.
    Clean up the gasket area well.
    Use grease on the new gasket.
    Reassemble. Do not over tighten the new bolts.

    You will not lose much coolant.

    Pour us a beer.
  5. Mickey Szypot, Feb 20, 2026
    #5
    Mickey Szypot

    Mickey Szypot New Member

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    Yes. Thank you for your input. That's the block heater mounting plate used on my 671. Seeing as the current block heaters were installed back in 2009, I think it might be time to replace both. ˆ'll make a plug to the exact size of the ID of the mounting plate. I'll let you know how I make out. I also will be replacing the two thermostats that control the block heaters. A fresh start. Beers on me.
  6. Dennis Elliot, Feb 20, 2026
    #6
    Dennis Elliot

    Dennis Elliot New Member

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    Mickey. I thought I needed to replace my block heaters on my J&T 671(s) just last week, but it seems it was a stuck pressure switch. My system is the same as yours ( I think). The heaters will shut down if it detects oil pressure (engine running) in case you forget to shut them off before starting. If you have this type of protection, please check this switch before doing anything else. I was not getting any power to the heaters, and I think it was because the switch was telling them that the engines were running. A very simple test, and it could save you a lot of money and work.
  7. Mickey Szypot, Feb 20, 2026
    #7
    Mickey Szypot

    Mickey Szypot New Member

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    Dennis, Thanks for your input. My block heaters work off of 110V. They plug into a receptacle located in the engine room. Both are connected to a thermostat that regulates the block temperature.
  8. Capt Ralph, Feb 20, 2026
    #8
    Capt Ralph

    Capt Ralph Senior Member

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    An Y wire harness that includes the thermostat. Another path for failures.
    Check out all before pulling that element assembly.
  9. Mickey Szypot, Feb 20, 2026
    #9
    Mickey Szypot

    Mickey Szypot New Member

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    Dennis, that's on my buckle list of things to do before I remove the block heater. Prior to replacing the current set of block heaters, I had an electrolysis issue with one of the thermostats. When my mechanic removed it, the bottom was missing. He could never figure out where it went. The engine has always runs consistently at 170 degrees under load. Unfortunately my mechanic and dear friend passed away. Keith Adams was his name and he was a diamomd in the rough old time Detroit Diesel mechanic.
    Long story short, we fixed the electrolysis issue by adding a zinc plate to the transom. My 1985 41' Viking didn't have one.
    Thanks for all your input.
    Mickey