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  1. #1
    Registered
    My Boats:
    1991 26' IMP - Twin 350s
    Join Date
    May 2011
    Location
    Austin, Texas
    Posts
    99

    Starting a dual engine setup with only one motor

    Greetings,

    I'm not sure if anyone can help without seeing schematics, but I'll try anyway.

    My IMP has twin 1991 EFI 5.7 Mercruisers. The starboard engine is at the machine shop getting rebuilt. Turns out the block is cracked. So while it's there, I want to startup the port engine to see if that block is cracked.

    With one engine out, and all of the wiring disconnected for the starboard engine, I'm wondering if it would do any harm to the port engine if I start it up (protecting all the loose wires and making sure nothing shorts of course).

    I guess my concern is around some vague memory of disconnecting a battery from a running engine and it damaging the alternator, or some such thing. Anyway, I thought I'd ask if anyone knows if this might cause damage to the electrical system. It's not a complicated boat, just your typical 1991 power boat with 350 twins. The port engine is all wired, it's just the starboard side that has the harness disconnected.

    Thanks in advance!
    Stan

  2. #2
    Registered
    My Boats:
    38 Fountain Lightning 575sci xr
    Join Date
    Jul 2007
    Location
    Oneida Lake NY
    Posts
    3,164
    Tie all the ground wires together of the missing engine together, you will be good to go.

  3. #3
    Platinum Member Platinum Member 1BIGJIM's Avatar
    My Boats:
    Pachanga Twin 502s Bob Mandra Cams
    Join Date
    Jun 2004
    Location
    Omaha, Nebraska. Boat on the Mighty Mo! Longest river in the USA!
    Posts
    1,653
    You don't need to start it to see if the block is cracked. Put water to the engine, if the oil goes up you have a cracked block. If the other cracked because it was not winterized properly, more than likely both blocks will be cracked.

  4. #4
    Registered
    My Boats:
    1991 26' IMP - Twin 350s
    Join Date
    May 2011
    Location
    Austin, Texas
    Posts
    99
    Thanks for the advice! Yeah I'll make sure those grounds are all tied.

    I want to start it for other reasons. It's been sitting for three years, and I want to make sure that the port motor is going to run before I put the starboard one back in. I need to know if I'm facing another big bill.

    It cracked because the intake was off and the engine filled with rain water and then froze. It was half torn down and covered, but the cover was leaking. The $1200 machine shop bill is my penance for being a half-assed idiot during repairs. The port motor I think was drained, but it was three years ago and I don't remember if I drained it or not. I may not have. There were lots of distractions back then. I guess the upside is that I'm getting lots of practice pulling and installing motors.

  5. #5
    Registered
    Join Date
    Apr 2009
    Posts
    221
    Expensive lesson

  6. #6
    Charter Member #232 Charter Member Audiofn's Avatar
    My Boats:
    1979 Formula 302, 88 Formula 311, 81 Donzi 18 2+3 with 454
    Join Date
    Oct 2000
    Location
    Manchester, MA USA
    Posts
    18,096
    Do you have dual or single battery switch? If you have dual just put the switch for that motor on and try and start it up.
    God is my Co-Pilot..... but I hit a mountain and had to eat him.

  7. #7
    Charter Member #232 Charter Member Audiofn's Avatar
    My Boats:
    1979 Formula 302, 88 Formula 311, 81 Donzi 18 2+3 with 454
    Join Date
    Oct 2000
    Location
    Manchester, MA USA
    Posts
    18,096
    I would put a new water pump in there first.
    God is my Co-Pilot..... but I hit a mountain and had to eat him.

  8. #8
    Registered
    My Boats:
    1991 26' IMP - Twin 350s
    Join Date
    May 2011
    Location
    Austin, Texas
    Posts
    99
    Yes I do have a dual battery switch. I switch that over too.

    Once I see that it runs, and everything is okay... then it gets a whole new raw water pump (impeller and housing), complete tuneup and oil change, and the stern drive gets new seals and bellows. What started this whole long string of events was that the impeller housing on the starboard motor cracked, shredding the impeller and sending all of the little bits of rubber into the oil cooler, causing the engine to overheat even after a new raw water pump.

    Of course before all of that happens, I have to get all of the 3 year old gas out of the fuel tank. I've been mixing it with a full tank of super, 5 gallons at a time in my truck. Slow process.

  9. #9
    Platinum Member Platinum Member seafordguy's Avatar
    My Boats:
    Cigarette - Cafe Racer
    Join Date
    Jun 2006
    Location
    Seaford, VA
    Posts
    3,020
    Quote Originally Posted by svonmiller View Post
    Once I see that it runs, and everything is okay... then it gets a whole new raw water pump (impeller and housing), complete tuneup and oil change, and the stern drive gets new seals and bellows. What started this whole long string of events was that the impeller housing on the starboard motor cracked, shredding the impeller and sending all of the little bits of rubber into the oil cooler, causing the engine to overheat even after a new raw water pump.
    AudioFn is trying to avoid this happening again by telling you to change the impellor first. It could shred instantly - even if it doesn't over heat things you don't want those chunks of crap throughout your coolers....

    I'd take the "better safe than sorry" approach considering the price of impellers.

  10. #10
    Registered
    My Boats:
    1991 26' IMP - Twin 350s
    Join Date
    May 2011
    Location
    Austin, Texas
    Posts
    99
    Good point, I wasn't thinking in those terms. I've already ordered the new pump and it'll be here on Tuesday, so I can definitely swap it out before a startup.


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