Yanmar 4JH3-THE

[Ip485]

I am finding at high revs. the temp warning comes on (3,000 rpm and above) and the temperature gauge would suggest the temperature has risen too high.


I have had the heat exchange washed through with Rydlyme (on the raw water side) the impeller is fine, the sea water strainer is clean, and the discharge good.


I havent checked or replaced the stat. so intending doing that. It has also been suggested the heat exchanged may need cleaning on the fresh water side.



Does anyone have nay suggestions or bet what is the cause. i am tempted to think the stat.



Thank you.

[Bruce Beard]

Our sailboat had a Yanmar 3JH4e engine, essentially the 3 cylinder version of your engine. It overheated under load from the time it was new...
I finally removed the heat exchanger (easy job) cleaned it and reinstalled it to solve the issue.
Yanmar suggests replacing coolant annually too.
Something had contaminated the cooling system in our engine and it took an annual cleaning and coolant replacement to keep things happy.
Check the exhaust elbow too. These can become restricted at the water injection point enough to impact cooling. It is dificult to see unless you remove the elbow and look from the exhaust side.

[Privilege]

Is the intake on the outside of the hull clean? The exhaust might be showing the water to be flowing freely but the engine isn't getting enough at higher revs.

Try the easiest things first!

[Bruce Beard]

Originally Posted by Privilege

Is the intake on the outside of the hull clean? The exhaust might be showing the water to be flowing freely but the engine isn't getting enough at higher revs.

Try the easiest things first!

Good advice trying simple first but it appears that they have gone over the intake side pretty well.
This series of Yanmars seem to have marginal cooling capacity by design. I’ve been aboard a number of boats that are fine until run hard. Then the alarm sounds. Some people never run the engines hard and are not aware that there is a problem!
Ours always had issues, from the time it was delivered brand new. 10 minutes at 2700 rpm’s (3000 red line) and the alarm would sound. Reduce throttle and everything would cool off. Once I cleaned the heat exchanger it could run wide open as long as you wanted without the gauge moving above about 180 degrees.
12 months later and it would be running hot again.

[MV Wanderlust]

A clean through-hull, clean strainer, good impeller, and clean heat exchanger that is cooling new, flushed coolant should be providing adequate incoming flow to cool the system. It's possible the thermostat is bad. They're easy to pop out and test or replace. It's also possible the manifolds/risers/elbows are clogged with rust or corrosion inside and preventing the raw water from flushing on through. If the thermostat proves to be fine, I would next look at the exhaust system. I did hear about, but never personally witnessed, one case of an unexpectedly heavy growth of marine critters in the muffler. Seems like a rare case, and probably the least likely, but possible.

[Ip485]

Thank you so far.


I should have added that I had the elbow reomoved and checked, it was as clean as a whistle. I also changed the coolant and used the Yanmar product.


I havent checked the intake, so that is a good suggestion, although the discharge does increase substantiallly with revs.


I havent got to grips with removing the heat exchanger. I have now been told by two engineers that removing the heat exchanger is a big job. This seems odd having read the manual which suggests otherwise so I am a little non plussed. My engineer suggested running Rydlyme through the exchanger which he did a few months back rather than remove it because he reckoned it would be a good days work (and I am not sure I had all the replacement gaskets at the time). I am becoming a little skepitcal that removing the exchanger is relatively straight forward but obvioulsy if it is the stat. that is defintiely a lot easier as I have ordered a new stat and the two gaskets required and as the stat has never been replaced it would seem to make sense to do so anyway.



My only thought is would a sticky stat. explain the symptoms. I suppose if it wasnt opening fully, but a bit, it would be ok at low revs., but would get hot at high revs. If the stat wasnt opening at all then it would overheat much quicker?

[Bruce Beard]

A faulty thermostat can absolutely cause overheat. I’ve seen some strange things attributable to faulty thermostats.
I haven’t seen the heat exchanger set up specifically but on our engine, the 3 cylinder version, it was pretty easy to get apart. Maybe he is worried about the element sticking in the bore. Ours was apart often enough that this was never an issue!
Again, the issue with ours was always on the fresh water/coolant side. Ridlyme would not have helped here.

[billknny]

Originally Posted by Ip485

I am finding at high revs. the temp warning comes on (3,000 rpm and above) and the temperature gauge would suggest the temperature has risen too high.


I have had the heat exchange washed through with Rydlyme (on the raw water side) the impeller is fine, the sea water strainer is clean, and the discharge good.


I havent checked or replaced the stat. so intending doing that. It has also been suggested the heat exchanged may need cleaning on the fresh water side.



Does anyone have nay suggestions or bet what is the cause. i am tempted to think the stat.



Thank you.

If you have kept the freshwater side of the engine full with a proper mix of water and corrosion inhibiting antifreeze, the chances of significant scale there are pretty low. Not zero, but low.

In addition to flushing the salt side of the heat exchanger, did you check EACH of the tubes for the possibility of a clog? If not, you should.

The thermostat is so easy to check, that should be your next test.

[Paul L]

I think this overheating when run hard at high rpm is pretty common on the 4JH3. Mine does.

[Ip485]

The thermostat is so easy to check, that should be your next test.

Do you mean in the usual way of removing it and using a bath - or are you thinking of some other way without removing it?



If it is removed and appears ok, I am inclinded to replace it anyway as most of the effort has been made (and I have ordered a new one anyway) and I guess since this one has served for 10 years that is reasonable going? Would you agree?

[CarlF]

My experience with a 4JH4-HTE is that the cooling is borderline. You've tried a lot of the things but here are a few more:

Follow your hunch and replace the thermostat even if it seems fine. They're cheap.

Replace the coolant with Yanmar spec coolant.

Replace the impeller. Also cheap.

Have you replaced the exhaust elbow recently? My 4JH4 says it's a "mandatory" replacement every 500 hours - even if it looks clean. Not sure about your engine.

Are there scoops on your water intake? And think about removing any screens on the intake - the internal strainer has a lot more surface area so it won't restrict the water.

A bigger job is to try to remove bends from the cooling line - especially 90 degree fittings. If it's a long hose run, increase the hose size.

[Bean Counter]

We had an overheat issue on a charter boat once, it was the pulley that drives the water pump, I thought it was a slipping V belt, but it turned out that the hole in metal pulley itself had enlarged enough to slip under high revs.

[daletournier]

Another thing to look at is your prop pitch. I have a Maxprop. When i first got the boat if i ran the engine above 2700 the temperature would creep up, not over heat but noticeably hotter. I worked out i was overpropped.

Ive since adjusted the pitch and now the temperature gauge stays just above 180 regardless of rpm. The downside is its not as economical as it was at cruising rpm.

[a64pilot]

Usually you can just remove the end cap on a heat exchanger and either rod out all of the tubes with a rifle cleaning brush and cleaning rod, or at least get a real good look and decide if you need to go any further.
Cap is almost always easy, access is what can make a simple job, hard.

Assuming you have the proper orange Yanmar coolant which is Dexcool, if anyone has ever added the regular green antifreeze, it is contaminated and can form a sort of gel that is difficult to completely remove, but aggressive flushing will remove all of it.
I install an automotive Prestone flush kit in the heat exchanger line for the water heater, and flush for a few minutes until after the water runs clear, then run it until it’s warm with just fresh water and flush again, repeat until you only see fresh, clean water. it’s really easy to do.

[svlamorocha]

Originally Posted by daletournier

Another thing to look at is your prop pitch. I have a Maxprop. When i first got the boat if i ran the engine above 2700 the temperature would creep up, not over heat but noticeably hotter. I worked out i was overpropped.

Ive since adjusted the pitch and now the temperature gauge stays just above 180 regardless of rpm. The downside is its not as economical as it was at cruising rpm.

Spot on. If the boat cannot reach rated rpm in gear then it may overheat just because you are making it work harder than the cooling system allows at X rpm. The trick is in the cubic shape of the curve that links HP demanded by prop vs rpm. You may be forcing the engine to deliver 2/3 of rated power with only 1/2 of cooling capacity.