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  1. #1
    Join Date
    Jun 2019
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    Cherkasy, Ukraine
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    5

    Exclamation 1996 Mercruiser 3.0 overheating at idle

    1996 Mercruiser 3.0 slowly overheating when idling, the temperature increases to over 200F and up, if I rev up the engine to 2000-2500RPM the temperature goes down close to 140F pretty fast.
    What I already did:

    • Replaced the impeller
    • Checked water supply

    Here is the video of water supply hose detached from the thermostat and running at 800RPM. Please let me know is the incoming water flow looks good enough



    Next things I'm going to do is to check the thermostat and exhaust. If the issue is clogged exhaust, any suggestions on how to clean it without removing from the engine?


    P.S. I'm new here so will appreciate any help

  2. #2
    Join Date
    Jul 2008
    Location
    Northboro, Ma, USA
    Posts
    5,342

    Default Re: 1996 Mercruiser 3.0 overheating at idle

    What you describe as your issue points to the water pump impeller in outdrive.

    Describe who did what and what was replaced and when.

    Low flow at idle and higher flow as rpm increases is 99% of the time the impeller.

    After it was replaced did someone start engine out of water with no water to drive while on trailer? If so you smoked the impeller.

    It is not a clog/blockage at this point.
    Jack
    Mass.

  3. #3
    Join Date
    Jun 2019
    Location
    Cherkasy, Ukraine
    Posts
    5

    Default Re: 1996 Mercruiser 3.0 overheating at idle

    Did you had a chance to check the video of incoming water flow I have at 800RPM? Is it looks okay or weak? I replaced the impeller during Spring maintenance, didn't run it without the water

  4. #4
    Join Date
    Jun 2019
    Location
    Cherkasy, Ukraine
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    5

    Default Re: 1996 Mercruiser 3.0 overheating at idle

    You probably right, I removed the thermostat yesterday, nothing changed, checked water channels for corrosion, they look pretty clear for a boat that used on fresh water. Now I am reading complains about Sierra impellers, people have too many problems with them, so I decided to recheck the water supply and I got about 1 gallon in 15 seconds at 1000-1200RPM, but then I decided to drop the RPM as low as possible (probably around 600) and water flow stopped. I think my Sierra impeller might be a problem so I ordered new OEM impeller kit together with the housing, will need to get the boat out of the water (this is the hardest part, that's why I hoped that the problem is somewhere in the engine) and change it up later this week. Will let you know if it fixes my overheating problem in case someone curious too.

  5. #5
    Join Date
    Jun 2019
    Location
    Cherkasy, Ukraine
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    5

    Default Re: 1996 Mercruiser 3.0 overheating at idle

    Click image for larger version. 

Name:	IMG_6834.jpg 
Views:	17 
Size:	58.9 KB 
ID:	20700

    My overheating on idle problem was caused by this little guy.

  6. #6
    Join Date
    Sep 2003
    Location
    New Tripoli, PA, USA!
    Posts
    16,284

    Default Re: 1996 Mercruiser 3.0 overheating at idle

    What is that? Inquiring minds want to know.

    Jeff

  7. #7
    Join Date
    Jul 2008
    Location
    Northboro, Ma, USA
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    5,342

    Default Re: 1996 Mercruiser 3.0 overheating at idle

    Quote Originally Posted by Kryptonics View Post
    Click image for larger version. 

Name:	IMG_6834.jpg 
Views:	17 
Size:	58.9 KB 
ID:	20700

    My overheating on idle problem was caused by this little guy.
    Good catch,

    Jeff,

    On the Gen 1 there is a short plastic "guide" tube I call it, that sticks into the impeller housing that the copper tube slide into and into the impeller housing.

    On the Gen 2 (in his picture) this is what is used. Just a different design for gen II

    This was most likely allowing exhaust gasses to enter the raw water stream, Speaking of which.
    Make sure that rubber DAM is in place if not or it is battered, REPLACE IT!.
    I will post a pic of the assembly in another post
    Last edited by kghost; 06-15-2019 at 10:17 AM.
    Jack
    Mass.

  8. #8
    Join Date
    Jul 2008
    Location
    Northboro, Ma, USA
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    Default Re: 1996 Mercruiser 3.0 overheating at idle

    Jack
    Mass.

  9. #9
    Join Date
    Jun 2006
    Location
    Spotswood, NJ, USA
    Posts
    4,845

    Default Re: 1996 Mercruiser 3.0 overheating at idle

    Considering the GEN II kit comes with everything needed, why only change the impeller?

  10. #10
    Join Date
    Jun 2019
    Location
    Cherkasy, Ukraine
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    Default Re: 1996 Mercruiser 3.0 overheating at idle

    My Sierra kit didn't include nor Water Coupling Assembly either impeller housing

  11. #11
    Join Date
    Jul 2008
    Location
    Northboro, Ma, USA
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    Default Re: 1996 Mercruiser 3.0 overheating at idle

    That is why it is always best to buy Mercruiser for most parts.

    It may cost a bit more but it will have everything you need in the kit and you know it will fit properly.


    Once you become familiar with what is what and what is needed then you may be able to go off brand for a specific item.

    But in the end. OEM!
    Jack
    Mass.

  12. #12

    Default Re: 1996 Mercruiser 3.0 overheating at idle

    Quote Originally Posted by Kryptonics View Post
    Did you had a chance to check the video of incoming water flow I have at 800RPM? Is it looks okay or weak? I replaced the impeller during Spring maintenance, didn't run it without the water
    I watched your vid. You're supposedvto hold the hose upright & see how much water comes out. It should shoot up about 6 inches or so. Just for future reference.

  13. #13

    Default Re: 1996 Mercruiser 3.0 overheating at idle

    Quote Originally Posted by kghost View Post
    That is why it is always best to buy Mercruiser for most parts.

    It may cost a bit more but it will have everything you need in the kit and you know it will fit properly.


    Once you become familiar with what is what and what is needed then you may be able to go off brand for a specific item.

    But in the end. OEM!
    When checking water flow thru the t-stat hose is it best practice to hold the hose upright or down so that the water runs into the bilge & guess at it?

    BAHAHAHAHAHAHA

  14. #14

    Default Re: 1996 Mercruiser 3.0 overheating at idle

    Quote Originally Posted by kghost View Post
    Wite*****,
    Actually you are wrong as usual.

    The correct method is not the one you describe. Wrong again!

    But unfortunately I aint gonna tell ya.


    Maybe see what OMC reccomends...
    Hi Ace! On an OMC Cobra why is it difficult to seperate the lower half from the upper drive half or vice versa even though the bolts have been removed, Ace?

    BAHAHAHAHAHAHA

  15. #15
    Join Date
    Sep 2010
    Location
    Jersey Shore
    Posts
    5,621

    Default Re: 1996 Mercruiser 3.0 overheating at idle

    2x's on the OEM pump kit. Get it with the housing and the lower seal carrier. The seal carrier is important to change.
    Chris
    Dockside Marine Services
    Jersey Shore.
    Mercruiser/Mercury Certified
    Let the insanity begin.

  16. #16

    Default Re: 1996 Mercruiser 3.0 overheating at idle

    Quote Originally Posted by Docksidemarineservices View Post
    2x's on the OEM pump kit. Get it with the housing and the lower seal carrier. The seal carrier is important to change.

    Listen to this one OEM water pump lasts twice as long.

  17. #17

    Default Re: 1996 Mercruiser 3.0 overheating at idle

    Quote Originally Posted by Dieter View Post
    Listen to this one OEM water pump lasts twice as long.
    I can rehab a water pump on a Cobra in an hour. How long for your Schmerc, Ace?

  18. #18
    Join Date
    Jun 2006
    Location
    Spotswood, NJ, USA
    Posts
    4,845

    Default Re: 1996 Mercruiser 3.0 overheating at idle

    Will all combatants go to their corners for a time out, lest you be punished

  19. #19

    Default Re: 1996 Mercruiser 3.0 overheating at idle

    OMG the adult like cartoons

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