Yanmar Engine issue


Surely there is an easy fix but…this has me stumped:

I have the Yanmar 4JH-3-HTE engine on my SM2000 (#404).  When I start it up cold, no water flows from the impeller to the heat exchanger.  A large air bubble forms in the bottom of the translucent hose that goes from the impeller and, of course, the engine begins to overheat if not shut down.  (Before I detected this problem, we overheated and melted the plastic muffler early into the Salty Dawg venture last month). 

Impeller recently replaced and working fine, water is flowing freely from the sea chest (e.g., when the heads are filled or the air conditioning is turned on), belts are new and snug.  There are no leaks, water or air, that I can detect anywhere along the path from the sea chest to and through the engine. 

But here’s the interesting part:

When I disconnect the hose from the heat exchanger (the hose that goes from the impeller to the heat exchanger), lay it down on the floor of the boat below the level of the impeller, and then start the engine, the water (and bubbles) starts flowing forcefully through the hose into the bilge—after which, with engine still running, I immediately slam the end of the hose back into the heat exchanger and tighten it down, the water continues to flow forcefully as it should (but no bubbles), the engine remains cool, and all is right with the world.

It seems to stay ‘primed’ if I shut it down and start it again after only a few hours.  However, if I start the engine up after a couple of days of not running the engine, I then have the same symptoms and cure it with the same solution.  

Thoughts? Ideas? THANKS!

Rick Grimes

S/V Rascal

Amel SM2K (#404)




I suggest that you do two things:

Open the boot on the sea chest side of the transmission oil cooler. This is inline the hose from the sea chest to the Yanmar water pump. Use a wet vac to suck all of the debris out of it.

Next, check the hose clamps at every connection between the sea chest and the water pump. Now would be a good time to replace them all. Do not tighten the hose clamps too tight on the transmission oil cooler body because the metal is soft and you will close the water paths through it.

I believe you have suction side blockage (most likely in the transmission cooler) and because of the suction side blockage you have air entering at a not-so-secure hose connection.

Please let us know if this guess on my part helps you solve your issue.

BeBe 387


     It is  likely to be as Bill suggests. I have had exactly the same problem which drove me mad for eighteen months.
The casing of the Johnson raw water pump in which the impeller is housed was worn by 3mm plus, resulting in the pump cavitating with air on start up.
Once I opened the face plate on the impeller housing the engine ran fine.
I couldn't work out what was wrong until I got a new pump and measured the inside of the housing  with a Vernier and it was plain to see the difference.
I had been in Brazil for a couple of years, in the Amazon and many of the tributaries which is what I believe caused it due to the heavy silt. I  also went through lots of impellers on the Onan and the Yanmar during this time.
New pump and it was fine.



Seafever SM 425

amelliahona <no_reply@...>

Hi Rick:

This is curious but the system has basically a suction side (thru hull  --> sea chest (including the manifold for the toilets/AC etc --> transmission intercooler --> suction side of the Yanmar engine driven sea water pump including the pump cover), and the pressure side (i.e. output side) as you know  goes from the pump to the engine oil cooler --> to the engine coolant heat exchanger --> to the turbo exhaust elbow --> etc. 

The output of the pump is dependent on there being both not too much suction required, and not too much head of pressure to pump against.  In this case I would guess that you have an air leak on the suction side in combination with some sort of (perhaps relative) obstruction on the suction side. Or, less likely in my opinion, there might be an obstruction on the outflow side (most commonly due to impeller pieces at the inlet of the engine oil intercooler).   Once the pressure is reduced on the output side by removing the hose, water can literally "flow" and the suction leak is relieved as not as much suction is required.

My trouble shooting usually follows the path of least resistance, and here is how I perceive that:

1.  Are you absolutely certain that the O ring is in the groove for the cover of the sea water pump (i.e. the O ring that seals the pump cover to the pump housing)?
2.  While you are checking that, visually confirm the impeller blades are in good shape.  You might even motor the engine thru a few cranks with the cover off the pump (sea chest closed) and verify the impeller is turning.  I had one once that delaminated the rubber from the metallic hub and while the hub turned the impeller was turning at a different rate.
2.  Have you inspected the transmission intercooler to see if it is clogged creating a suction side negative pressure that may be resulting entrainment air leak at any of the joints between the sea chest and the sea water pump inlet.
3.  Have you tried shutting off the manifold to the toilets, AC etc to see if perhaps the air entrainment is coming from there. 
4.  Are you sure there isn't partial blockage of the thru hull, resulting in greater negative pressure on the suction side.
5.  Have you verified that the inlet to the engine oil intercooler (tucked between the turbo and engine block) is clear (this is harder to get at so that is why I have listed it last.)
6.  How many hours on the engine/sea water pump, I suppose if it is a high time engine there could be a water pump shaft seal leak allowing air entrainment.  If all else fails swap out the sea water pump with your new spare one.

Just some ideas,  let us know what you eventually find. 


Gary S. Silver
s/v Liahona    Amel SM 2000   On the hard for two more weeks Puerto Del Rey, Puerto Rico
Hull # 335 

Rick Grimes

Thanks all for the quick and thoughtful responses to my engine woes!  I have done only a few of the steps suggested. I will immediately begin to explore and test all the options and let you know when and how I identify and fix the problem. 



Rick & Linda Grimes
S/V "Rascal"
SM2K (#404)
Sent on the run from my iPhone


Problem solved! Thanks all for your thoughts and ideas on dealing with the air leak somewhere around my Yanmar engine that prevented the proper flow of raw water.  With the advice and help of Steve Davis ("Aloha"), I went first to the most likely place ("Are you sure the O ring is properly seated in the impeller housing?" asked Gary Silver).  When I last changed the impeller in the middle of the North Atlantic last November, I neglected to also check and change out the O ring.  When I pulled the cover of the housing, it was fairly corroded so I cleaned it up, lightly sanded the surface, and replaced with a new O ring. 

Further, while reading the instructions for the use of the "Speed Seal" cover (installed by the previous owner), I saw that I was supposed to have a white plastic washer underneath a metal washer--both--not one or the other. The plastic washer was missing and found later on the floor underneath the engine.  Also the inside of the Speed Seal cover was pretty worn.  Thus, I decided to replace the speed seal with the original cover (happily left among spare parts by the previous owner). 

Though the impeller was in good shape, I went ahead and changed it out and then put everything back together. During this process, of course, all the water had drained from all the hoses and the pump. So the real test (to see if the impeller housing and cover was the source of the air leak) was to start the engine and see if it self-primed--and it did! Works like new now. 

All that said there were several thoughts on other potential sources of the air leak and while, so far, it seems to be the impeller housing, I will next check all those areas to ensure they are and remain healthy (e.g., the boot area mentioned by Bill Rouse and other areas where I'm likely to spring a leak).    

I posted some photos here:


Rick and Linda Grimes

"Rascal" SM2K (#404)

Sun Bay Marina, Fajardo, PR



On checking other connections for the cooling water. A specialist in Malta who supplied me with a spare transmission cooler told me that these coolers almost never fail unless the hose connectors are over-tightened, cracking the solder.


I have read many post of the subject of the Amel floating ground system and how this applies to the engine, and starter.  I have also heard that the Perkins M80T was the only engine made for a floating ground.  So I would like Amel owners, who have repowered their Super Maramu with Yanmar, 4JH80, how they addressed the floating ground issue.  Was the engine modified  at the factory or did the installer make modifications to the starter, alternators etc.?
Thank you

John Rogers
SM #105
Deltaville, VA


As an additional note to my March 8 post, I have installed the Yanmar 4JH80 with isolated grounds to all the sensors and alternators.   However, I have found that beginning with this year Yanmar is making their engines with an electronic ignition which requires constant power and a ground to the engine block which ob course violates Amel's isolated ground design.  The Boatyard mechanic is telling me that an isolating prop shaft coupling by Global Rubber will prevent any current from going into the water and will solve any corrosion problems.  Have anybody encountered this issue?