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Mercruiser 260 Overheating

Old 05-28-2020, 04:04 PM
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Default Mercruiser 260 Overheating

I have a 1987 Cobalt br21 yesterday she started to overheat, immediately kept the temperature level safe and got at home. I went through and did some basic research and started to address the most simple immediate issues. All the parts of external lower unit, hoses and belts were good, checked the impeller, and it was still intact and operational. Looking for the next step to go. Any help would be gratefully appreciated, thank you so much!
Old 05-28-2020, 04:35 PM
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A "few " questions:
Is it RWC or FWC?
Do you use the boat in SW or just fresh?
Did it only overheat at higher RPM or if you start it cold and leave it in N will it eventually overheat after it fully warms up?
What temp did it get up to & did the alarm sound?
Could the overheat have been temporary due to a bag or weed over the intake? Is there any growth over the intake?
Was the L/U in the sand at all?
Oil Level good?
Check the t-stat in a pot of warming water with a candy thermometer to see when it opens up compared to it's spec temp? Is the t-stat housing clogged or clear, and are the spring-loaded ball bypass poppet valves on either side of that housing intact and free to open under water pressure ?

When was the last time the risers were replaced, and if the engine ifs RWC, the exhaust manifolds too? Generally,( in SW) they should be replaced as preventative maintenance every ~3-5 years, though many people prefer to take their chances by waiting until their water passages clog up or rust through to allow water into the engine.

Note that anytime you can't keep your palm on top of the risers indefinitely at any rpm , that is a clue RW flow there is insufficient. Sometimes it can just be a problem on only 1 side.
Old 05-28-2020, 04:48 PM
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Alpha drive? How did you verify that the impeller was functioning? The gen one Alpha has a pretty small hole for the water to pass thru and corrodes easily. Pull the drive and clean it out perhaps?
Old 05-28-2020, 04:59 PM
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Impeller (I would do a new one anyway), thermostat, and water pump. That’s where I would start.
Old 05-28-2020, 05:07 PM
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I say always replace the impeller when it’s out, inspect it and throw it away. New in.
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Old 05-29-2020, 03:56 AM
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Originally Posted by TTaxi View Post
A "few " questions:
Is it RWC or FWC?
Do you use the boat in SW or just fresh?
Did it only overheat at higher RPM or if you start it cold and leave it in N will it eventually overheat after it fully warms up?
What temp did it get up to & did the alarm sound?
Could the overheat have been temporary due to a bag or weed over the intake? Is there any growth over the intake?
Was the L/U in the sand at all?
Oil Level good?
Check the t-stat in a pot of warming water with a candy thermometer to see when it opens up compared to it's spec temp? Is the t-stat housing clogged or clear, and are the spring-loaded ball bypass poppet valves on either side of that housing intact and free to open under water pressure ?

When was the last time the risers were replaced, and if the engine ifs RWC, the exhaust manifolds too? Generally,( in SW) they should be replaced as preventative maintenance every ~3-5 years, though many people prefer to take their chances by waiting until their water passages clog up or rust through to allow water into the engine.

Note that anytime you can't keep your palm on top of the risers indefinitely at any rpm , that is a clue RW flow there is insufficient. Sometimes it can just be a problem on only 1 side.

Thanks for responding so quickly, the boat has always been in freshwater upstate NY, I believe its RWC (no coolant/antifreeze) it did not overheat at high rpms we were drifting shoreline where i would only have to run it for a few minutes to idle back out. The high temp buzzer came on (or I noticed at least) at about 200°F. The lake we were in was free of weeds or debris however I do have to go through a shallow sandy channel to get out to the lake. Oils were just replace this weekend and levels are good. I will check the thermostat when I get home, could it be an obstruction in the circulation system or the belt driven water pump?


Old 05-29-2020, 04:15 AM
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Originally Posted by IMPLiberty View Post
Impeller (I would do a new one anyway), thermostat, and water pump. That’s where I would start.
I will replace the impeller regardless now that i have the LU apart.

Old 05-29-2020, 04:20 AM
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Originally Posted by alwhite00 View Post
Alpha drive? How did you verify that the impeller was functioning? The gen one Alpha has a pretty small hole for the water to pass thru and corrodes easily. Pull the drive and clean it out perhaps?
It is an Alpha One, i am not certain of the gen. I verified that the impeller was intact, the drive looked good and hole did no seem plugged.
Old 05-29-2020, 04:54 PM
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Here is my Bravo impeller. It is “intact” but as you can see it’s not sealing.
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Old 05-29-2020, 06:04 PM
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Originally Posted by Zreed47 View Post
I will replace the impeller regardless now that i have the LU apart.
I just replaced a near perfect “looking” impeller because after going through the effort it’s worth replacing.
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Old 05-30-2020, 01:13 AM
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A marine engine will overheat at idle RPM even with a brand new perfect impeller if the RW pump has scored wear-surfaces on the housing so the impeller won't develop enough suction to draw and push enough water . The clue on that cause is that at higher rpm , when the impeller vane tips are no longer touching the housing and the pump is then acting as a centrifugal pump, and the water flow is being forced into the intake, the engine can cool right back down with sufficient RW flow.

In some cases an errant crushed o-ring pump seal might do the same.

How did you get the engine to cool down for the trip back?

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