Mercruiser 3.0 Leaking Water On Top of Manifold

Joined
Apr 7, 2024
Messages
4
Dear fellow boaters,

This past fall I recently acquired a boat equipped with a Mercruiser io 3.0 engine that initially had starting issues. After conducting a thorough examination, I discovered a bent intake valve and promptly had the valve seat reseated at a trusted machine shop. Following this, I replaced all necessary gaskets and rebuilt the carburetor to ensure optimal performance.

Initially, after reassembly, I successfully managed to start the engine and achieve smooth idling while using muffs in my garage. I never had a chance to get it out in the water before winterizing it as it was end of November.

However, as I prepared to take the boat out for a run after winterization today, a new issue emerged. Upon starting the engine and idling on muffs, I observed water seeping out of the plug hole on the back of the boat with fuel/oil. Upon closer inspection, I determined that the leak appears to be originating from the area near or above the gasket connecting the head to the exhaust/intake manifold. The water is running down the back of the engine and along the side, indicating a potentially serious issue. Could it be the manifold gasket? Could the pressure in the exhaust be pushing the water out to make it look like it is boiling? As seen in the video, could water be leaking past the threads of the head bolt?

To provide comprehensive context, I have attached detailed photos and videos for your review.

While considering potential causes such as improper sealing, damaged gaskets, uneven torque application, and the possibility of a cracked manifold, I am reaching out to this community for further guidance. Despite my efforts, I have been unable to pinpoint the exact source of the leak and resolve it effectively before I take it all completely apart.

In light of this, I would greatly appreciate any insights, suggestions, or troubleshooting tips that could help me identify and rectify this persistent water leak issue. Your collective expertise and advice would be invaluable in getting my boat back on the water and running smoothly.

Thank you sincerely for your time and assistance.
 

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Bondo

Moderator
Staff member
Joined
Apr 17, 2002
Messages
70,953
Could it be the manifold gasket? Could the pressure in the exhaust be pushing the water out to make it look like it is boiling? As seen in the video, could water be leaking past the threads of the head bolt?
Ayuh,..... Welcome Aboard,..... Can't be the intake manifold gasket, No water behind it,....
Head bolt is possible, I use a teflon paste on mine, were yer's sealed at all,..??

Is that hose right there leaking,..??
The t-stat housing,..?? or it's hoses,..??
 

bbook83

Petty Officer 2nd Class
Joined
Nov 6, 2004
Messages
176
There is a drain plug on the underside of the manifold. Is it coming from there?
 
Joined
Apr 7, 2024
Messages
4
Ayuh,..... Welcome Aboard,..... Can't be the intake manifold gasket, No water behind it,....
Head bolt is possible, I use a teflon paste on mine, were yer's sealed at all,..??

Is that hose right there leaking,..??
The t-stat housing,..?? or it's hoses,..??
I used permatex high temp thread sealer on the bolts. The hose is not leaking or any of the hoses.
 
Joined
Apr 7, 2024
Messages
4
How did you winterize?
1)Put stabilizer in gas
2) Run the engine and spray fogging oil into carb and kill it.
3) After it has cooled take drain plug out of the manifold and block
4) Once drained put plugs back in and take hose clamps off of tstat
5) pour antifreeze down the hose until comes out tstat and pour it into the exhaust hose and the intake hose.
6) Open drains again for a little bit and replug it
7) change oil and oil filter
8) change oil in outdrive
9) take out batteries
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
Staff member
Joined
Jul 23, 2011
Messages
49,536
1)Put stabilizer in gas
2) Run the engine and spray fogging oil into carb and kill it.
3) After it has cooled take drain plug out of the manifold and block
4) Once drained put plugs back in and take hose clamps off of tstat
5) pour antifreeze down the hose until comes out tstat and pour it into the exhaust hose and the intake hose.
6) Open drains again for a little bit and replug it
7) change oil and oil filter
8) change oil in outdrive
9) take out batteries
did you probe the drains in #3
 

dubs283

Vice Admiral
Joined
Jul 27, 2005
Messages
5,322
With the engine running on muffs use a mirror and light to inspect under the intake/exhaust manifold near the area where the head mounts to the block and you might locate the source of the leak
 
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