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  1. #1

    Default Mercruiser 5.7 Hot Water Heater Hook up

    I am looking for a diagram that shows the correct location to tap off for a hot water heater (heat exchanger) on a Mercruiser 5.7 raw water cooled engine. I have seen a few different hook ups, but wondering which is correct. Anything in print on this? Thank you

  2. #2
    Join Date
    Dec 2006
    Chaumont Bay, N.y., USA

    Default Re: Mercruiser 5.7 Hot Water Heater Hook up

    Ayuh,... The "Hot" line to the heater has to come outa the intake manifold, 'n the return "Cold" line goes into the plugged hole in the circulating waterpump...
    Any Grease is Better,... Than No grease at All....

  3. #3
    Join Date
    May 2010
    DC Burbs/Maryland Eastern Shore

    Default Re: Mercruiser 5.7 Hot Water Heater Hook up

    Just my two cents, but I would highly recommend converting to closed cooling before connecting the engine to a water heater. Will save you the possibility of a giant headache.
    Practicing the ancient art of ren-ching

  4. #4
    Join Date
    Mar 2009
    Portland, Oregon,

    Default Re: Mercruiser 5.7 Hot Water Heater Hook up

    This may help you understand why and where you provide the supply and return.

    Supply or mild pressure side:
    The Engine Circulating pump provides a mild positive pressure to the engine coolant side via the Thermostat holding back coolant on an "As Needed" or "On Demand" basis.
    Therefor the supply side must come from Upstream of the Thermostat....
    ( I.E., one of the ports in the Intake manifold, as mentioned)

    This may involve a "T" fitting where the temp sender is currently located..... unless your intake manifold provides a second port.
    No other supply/source will work as effectively (and DO NOT tap into an exhaust manifold for an "open" system!!!!!)

    Return or Suction side:
    The Starboard-most port on the Engine Circulating pump will be your return (this provides a mild Negative pressure).
    Again, no other source will work as effectively.

    Purging of air for a Closed Cooling system:
    Once connected up, use a Tempo Oil Boy to pull a suction on the supply side hose.
    This will draw coolant in reverse direction, and will allow you to purge most air from the Water Heater H/E tube (or cabin heater core) portion of the new system.
    Keep topping off the H/E with coolant as you do this.
    This generally saves the Warm/Cool cycles that are otherwise needed to purge the system of air.

    Warm your engine up before attempting to remove these fittings or plugs.
    A bit of heat may help you loosen these up.... especially if raw water cooled.
    You can always complete the work once things have cooled down again.

    I agree with o2batsea regarding doing this on an "Open System".
    You may have some corrosion issues down the road.... particularly if the WH h/e tube is aluminum.

    If you were to convert to Closed Cooling, you will also have some initial issues to over-come re; engine rust scale and the E/G/h2o side of the H/E tube bundle.
    It's doable.... just an FYI on that!

    Last edited by RicardoMarine; 04-28-2011 at 09:05 AM.
    Rick ... aka Ricardo..... AQ series Volvo Penta repair
    Portland, Oregon
    28' SDN F/B w/ twin Volvo Penta 5.7L DP's

    Please... no PMs! Post your questions on the forum.

  5. #5

    Default Re: Mercruiser 5.7 Hot Water Heater Hook up

    Thanks guys. I was considering doing the closed cooling kit, this engine has never been in salt water and has only 140 hrs on a fresh water lake. it looks brand new. thanks for the answers

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