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Old 16-05-2021, 08:33   #1
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Overheat at low rpm - 3JH4E on Hunter 38

Chugged down the ICW from Fort Pierce to Lake Worth yesterday in my new to me 2007 H38 and experienced odd overheating.

I cannot run over 1,700 rpm for any length of time (> 10 mins dependent on engine speed) without getting an overheat alarm. When the alarm goes on, a couple mins at low (1,100 or so) rpm resets it.

Almost continuous whisks of white out the exhaust which I believe to be steam not smoke consistent with overheating.

Luckily we had the headsail out most of the way and made 4.5-6.6 kts at 1,600 rpm.

I checked the sea strainer, clean. Impeller, new in Dec, looks great. No collapsed hoses, no kinks or otherwise obvious hose issues. There’s water spitting out the exhaust. No obvious water leaking from fresh water pump. V belt was rather loose, I tightened it.

Coolant tank a little above the “Low” mark with no oil in in. Oil nice and clean and slightly above the high mark (not done by me!). No air filter element in silencer.

Other than the belt, nothing obvious. I’m stumped.

Any ideas?

Now on the hook north Lake Worth hoping to sail to Port Everglades tomorrow.
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Old 16-05-2021, 09:20   #2
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Re: Overheat at low rpm - 3JH4E on Hunter 38

Bad pressure cap (replace w/specified psi rating), thermostat not opening are things not on your list that quickly come to mind. I'm doubting that the cap has ever been changed and would be my #1 easy change out. Should be able to find a replacement at an automotive store for a quick fix.

Any coolant leaks at the neck? This could also cause low psi in the coolant system and cause it to boil at a lower temp.
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Old 16-05-2021, 10:05   #3
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Re: Overheat at low rpm - 3JH4E on Hunter 38

Additional things to check:

Sea strainer through-hull not blocked
Exhaust elbow clear
HE tube stack not blocked

At the rpm your running I'm thinking exhaust elbow. Do you have an infrared temp gun? If not, get one and go through the cooling system. Yanmar recommends changing the coolant every 2 years. I'd take that recommendation to heart. Look at the galvanic smorgasbord in the HE end cap construction. 5 different metals and the most vulnerable is the aluminum alloy of the manifold/heat exchanger.
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Old 16-05-2021, 10:45   #4
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Re: Overheat at low rpm - 3JH4E on Hunter 38

No coolant leaks anywhere that I can see, and the coolant level is not changing.

I have several IR remote thermometers, of course none of them with me.

Maybe I can uber to an auto store.

I think the next step is check the h2o output at the mixing elbow and see if that is within spec.

I assume blockage at the through hull I need to dive under the boat to check?
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Old 16-05-2021, 10:54   #5
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Re: Overheat at low rpm - 3JH4E on Hunter 38

Quote:
Originally Posted by flightlead404 View Post
>>>>>>>>
>>>>>>>>>>>>>>>>>>>>


I assume blockage at the through hull I need to dive under the boat to check?



Take a dinghy foot pump, connect to a hose to your inlet thru hull, pump.


Has worked for me in the past. Good luck.


May not be your issue if you're seeing water out the exhaust. From everything I've read about Yanmars (I do not have one) the exhaust elbows are often the culprit.
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Old 16-05-2021, 10:55   #6
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Re: Overheat at low rpm - 3JH4E on Hunter 38

I assume blockage at the through hull I need to dive under the boat to check?

Not if you have a long enough hose to get above the water line and a long enough dowel or threaded rod to punch through.
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Old 16-05-2021, 10:59   #7
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Re: Overheat at low rpm - 3JH4E on Hunter 38

Quote:
Originally Posted by Stu Jackson View Post
Take a dinghy foot pump, connect to a hose to your inlet thru hull, pump.


Has worked for me in the past. Good luck.


May not be your issue if you're seeing water out the exhaust. From everything I've read about Yanmars (I do not have one) the exhaust elbows are often the culprit.
Good idea on the foot pump! With Yanmars it depends on what exhaust elbow was used. The roundish cast iron ones are infamous. On the OP's 3JH he might have the boxed shape bronze design. They will outlast the boat.
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Old 16-05-2021, 11:10   #8
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Re: Overheat at low rpm - 3JH4E on Hunter 38

Sounds like you aren’t getting enough raw water through. The little that makes it through is flashing to steam.

Definitely check the exterior thru-hull strainer.
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Old 16-05-2021, 12:22   #9
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Re: Overheat at low rpm - 3JH4E on Hunter 38

Check the raw water pump carefully. Often the cover is worn enough to seriously affect flow even with a new impeller. Also, some pumps have a wear plate behind the impeller which can also wear enough to affect flow.
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Old 16-05-2021, 12:28   #10
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Re: Overheat at low rpm - 3JH4E on Hunter 38

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Originally Posted by C420sailor View Post
Sounds like you aren’t getting enough raw water through. The little that makes it through is flashing to steam.

Definitely check the exterior thru-hull strainer.
+1 for this from me, but I suspect the exhaust elbow.
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Old 16-05-2021, 13:18   #11
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Re: Overheat at low rpm - 3JH4E on Hunter 38

Maybe your heat exchanger needs a good cleaning?
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Old 16-05-2021, 13:34   #12
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Re: Overheat at low rpm - 3JH4E on Hunter 38

If you have your pyrometer, shoot the temp on the thermostat as she overheats. If the the temp is between 165 and 169, it's not your thermostat. It should stay in this range. If the temp goes above 170 at the thermostat, replace it. At the same time, shoot the temp on the mixing elbow. If it's higher than 80 or so, that's your problem. The only way to check the heat exchanger is to open the end cap and inspect it. The last thing would be the actual sea water pump itself is weak from age.
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Old 16-05-2021, 13:36   #13
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Re: Overheat at low rpm - 3JH4E on Hunter 38

One other thing.....you might have to burp the air out of the coolant side by opening the cap and pouring in more coolant.
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Old 17-05-2021, 09:14   #14
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Re: Overheat at low rpm - 3JH4E on Hunter 38

I had a 1 GM for years with exactly the same problem and I did all the things suggested but for years whenever I had to run up to 2500 to get through a bridge against the tide, for instance, I'd overheat with the same result: drop the hook, count to 100, start again and listen to the buzzer for a couple minutes until the engine cooled and carry on at low RPMs. I finally started taking the entire system apart (in exasperated disgust) one piece at a time starting with changing the impeller (again) then each piece apart, check it or replace it, I even bored the brass elbows out slightly larger until I finally found the culprit: a tiny mussel had lodged itself in the elbow downstream of the filter. Enough water would get through for small RPMs, not enough for high revs which you never use unless you really, really needed to which was when the engine would overheat.
Removed the mussel, never had the problem again. Not something you'll find in the manual but made perfect sense once I found it. Not sure how it made it past the intake strainer and all the elbows and the strainer to lodge itself after the strainer but pretty sure you have a minor blockage somewhere.
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Old 17-05-2021, 09:15   #15
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Re: Overheat at low rpm - 3JH4E on Hunter 38

Often when an impeller goes bad (yours was replaced in Dec, did you replace it or was it the P.O.) pieces get stuck in the heat exchanger. easy enough to pull the end covers off and have a look. Also a good time to check the zincs.

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