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Old 17-05-2021, 09:26   #16
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Re: Overheat at low rpm - 3JH4E on Hunter 38

Since it runs ok at lower RPM,s then it has "insufficient" cooling at the higher power outputs.

If I were to put my money on it, I would say the exhaust elbow.

On a Yanmar, the exhaust elbows should be treated like a "maintenance item" requiring replacement every xxx years.

That followed by a heat exchanger partially clogged on the raw water side with sea life, old impeller parts, old zincs .

If the above are OK, then the other usual raw water system suspects: clogged through hull inlet, suction hose between between through hull and pump collapsing.

When you look at the water pump">raw water pump impeller make sure it is not free-wheeling on the shaft.

On the coolant side, thermostat only partially opening, and the oddball failure of the coolant pump impeller slipping on the shaft.

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Old 17-05-2021, 09:29   #17
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Re: Overheat at low rpm - 3JH4E on Hunter 38

Time to inspect and clean or replace heat exchanger (not cheap).
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Old 17-05-2021, 09:32   #18
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Re: Overheat at low rpm - 3JH4E on Hunter 38

I brought my heat exchanger to a radiator shop, had it hot tanked, pressure tested. Primed and painted it...good as new.
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Old 17-05-2021, 13:06   #19
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Re: Overheat at low rpm - 3JH4E on Hunter 38

I had similar problem, it was a clogged inlet hose from the strainer to the raw water pump! Clogged with Barnacles! replaced hose and no problems with running fast or slow.

I expect the tiny seeds of the barnacles can get through the strainer to start growing in the hose when boat sits and little or no engine use for long periods.

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Old 17-05-2021, 14:31   #20
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Re: Overheat at low rpm - 3JH4E on Hunter 38

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Originally Posted by captmikem View Post
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.

M
This: also pull the impeller again or pull the outlet hose off the pump & examine the pump outlet very carefully for old impeller blades.
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Old 17-05-2021, 15:06   #21
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Re: Overheat at low rpm - 3JH4E on Hunter 38

Might have come in through the strainer as a little baby mussel and then grew up inside your cooling system.

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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, 15:07   #22
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Re: Overheat at low rpm - 3JH4E on Hunter 38

It will be a simple calcium build up in the raw water side of the cooling system. Watch a few Barnacle Buster Youtube videos and decide which one is applicable to your system. Use their product or equivalent acids from other brand products that contain Phosphoric acid. Phosphoric acid is what works best.
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Old 17-05-2021, 15:07   #23
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Re: Overheat at low rpm - 3JH4E on Hunter 38

My Hunter 46 Yanmar engine had a similar issue when operated at very high rpm, during an engine inspection. Mechanic said it a slightly fouled heat exchanger that needed to be cleaned or replaced (not cheap). However, it does not run hot at my normal cruising rpm so I am deferring this maintenance until I plan a long trip.
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Old 17-05-2021, 15:46   #24
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Re: Overheat at low rpm - 3JH4E on Hunter 38

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Originally Posted by Ramona View Post
It will be a simple calcium build up in the raw water side of the cooling system. Watch a few Barnacle Buster Youtube videos and decide which one is applicable to your system. Use their product or equivalent acids from other brand products that contain Phosphoric acid. Phosphoric acid is what works best.

Good reminder. When we bought our boat in 1998 the PO said it overheated at WOT.


First thing I did was remove the HX and found the inlet rw port 3/4 blocked by salt. Easy fix. Good luck.
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Old 17-05-2021, 16:42   #25
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Re: Overheat at low rpm - 3JH4E on Hunter 38

I have this same engine * love it. A line wrapped the prop on mine and caused similar overheating symptoms
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Old 17-05-2021, 17:07   #26
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Re: Overheat at low rpm - 3JH4E on Hunter 38

The OP says water is gushing out the exhaust. Doesn't take much seawater to cool so to me it is unlikely anything wrong in the seawater side. And if the exhaust elbow was clogged the engine would have power issues and the water wouldn't be “gushing”

The only stated issue is a loose belt. This is the item to me to check first by replacing because in my experience once it has loosened it is not reliable.

Btw - i had the heat exchanger off the engine last year. Yanmar 4JH3, 3200 hours, 20 years old. Tube anf exhaust elbow were clean. Don’ jump to the hard choices before checking the easy ones.
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Old 18-05-2021, 00:13   #27
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Re: Overheat at low rpm - 3JH4E on Hunter 38

Quote:
Originally Posted by sail sfbay View Post
My Hunter 46 Yanmar engine had a similar issue when operated at very high rpm, during an engine inspection. Mechanic said it a slightly fouled heat exchanger that needed to be cleaned or replaced (not cheap). However, it does not run hot at my normal cruising rpm so I am deferring this maintenance until I plan a long trip.

Watch some Barnacle Buster Youtube videos.
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Old 18-05-2021, 01:35   #28
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Re: Overheat at low rpm - 3JH4E on Hunter 38

Quote:
Originally Posted by sailorboy1 View Post
The OP says water is gushing out the exhaust. Doesn't take much seawater to cool so to me it is unlikely anything wrong in the seawater side. And if the exhaust elbow was clogged the engine would have power issues and the water wouldn't be “gushing”.........


The OP said "There’s water spitting out the exhaust.". He said nothing about 'gushing'. Also this boat is 'new to him' so he has no idea how much water should be coming out.

While there are several reasons for overheating, I strongly suspect the salt water side is at fault and most likely either a clogged elbow or a clogged heat exchanger. Both items get little attention in the time before a boat goes onto the market.
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Old 18-05-2021, 06:27   #29
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Re: Overheat at low rpm - 3JH4E on Hunter 38

Took a slip in Ft. Lauderdale to fix this.

I'll order a new f/w pump even if I don't need it, its a good spare to have.

Will de-scale the heat ex and look for bits of impeller (the PO did it) and refresh the coolant.

Will also check the raw water pump throughput today.

Thanks for the advise.
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Old 18-05-2021, 06:40   #30
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Re: Overheat at low rpm - 3JH4E on Hunter 38

What's your temp gauge reading? How fresh is your anti-feeze coolant? When was the thermostat last changed? If old antifreeze then system needs to be flushed including draining and flushing the block. JMHO
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