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Yamaha 8hp 4-stroke cooling issues

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Wil_K

New member
Hi all, I have a 2006 Yamaha 8hp 4-stroke with a difficult to pinpoint cooling issue. At mid-high RPM with the engine under load, flow disappears from the telltale. I can rev the motor to the limiter in neutral and water flow remains perfect, the issue only occurs in gear and only at higher revs. When I back off the throttle, flow returns and it feels hot.

Waterways appear clean and the impeller is new. I was thinking it could be the head gasket, but compression is an even 180psi, oil is clean, and the motor runs strong. It seems like exhaust pressure in the cooling system could be the cause. I’m hesitant to remove the head just to check the gasket.

Would it be possible that the head gasket is still bad with good compression readings? Or is there another part of the motor that could be causing exhaust leaks into cooling passages? Any other ideas?

Cheers
 
How did it cool for you last season ?----Are you testing in a bucket with water 8" above the impeller ?-----Or short shaft motor on a 20" transom ?
 
racerone said:
How did it cool for you last season ?----Are you testing in a bucket with water 8" above the impeller ?-----Or short shaft motor on a 20" transom ?
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I bought it cheap recently with the cooling issue known. I tested in a deep bucket and at sea (transom height is correct), as the only time I could replicate the problem was under load. No issues when running in the bucket.
 
If the over temperature alarm is not sounding (with the motor also going into RPM reduction mode) you don't have a cooling problem.

Tell tale water has nothing whatsoever to do with the water that is going through the block and head to keep the motor cool.
 
There's no temp alarm, or rev limiting feature. I understand the telltale is just one of the exit points for cooling water, the point is the telltale is working perfectly revving to maximum rpm in neutral, and at low rpm in gear, there is no overheating in these instances. The telltale disappears at high rpm in gear. When revs are lowered, it returns and is much hotter than it should be indicating coolant has not been flowing through the block.
 
In post #1 you mention the impeller is new. Did you replace the impeller only or put a “water pump repair kit” in it?
 
What is the model of the motor?

Folks hide their model identity more securely than NSA guards top secret documents.
 
Wil_K said:
There's no temp alarm, or rev limiting feature. I understand the telltale is just one of the exit points for cooling water, the point is the telltale is working perfectly revving to maximum rpm in neutral, and at low rpm in gear, there is no overheating in these instances. The telltale disappears at high rpm in gear. When revs are lowered, it returns and is much hotter than it should be indicating coolant has not been flowing through the block.
Click to expand...
Where did you get this information from?
 
Model number is not known as the original transom clamp sticker is missing. It is most likely an F8MH approx 2006. The owners manual has no mention of a warning alarm system or overtemp protection system.
 
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