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Old 19-05-2021, 09:13   #46
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Re: Overheat at low rpm - 3JH4E on Hunter 38

Does your engine have an oil cooler?
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Old 19-05-2021, 09:18   #47
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Re: Overheat at low rpm - 3JH4E on Hunter 38

Water flow at idle looks like what I would expect.

Iíd expect more at 2000rpm...but every diesel make/model is different.

Those numbers look pretty good, assuming theyíre representative of water temp.

Remember, diesels donít like to run cold. Obviously overheating is bad, but a hot diesel is a happy diesel.
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Old 19-05-2021, 09:18   #48
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Re: Overheat at low rpm - 3JH4E on Hunter 38

Hard to tell w/the shorter time of 2nd vid if they are the same flow, but they look similar and looks like an ok amount.

So what happens now if you run it in gear at 2K for the same time, does it still overheat?


Edit: looked through the thread quickly again and will ask if the prop is clean as I didn't see this mentioned.
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Old 19-05-2021, 10:32   #49
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Re: Overheat at low rpm - 3JH4E on Hunter 38

Prop is clean

Qualitatively ďmoreĒ water exits at 2,000. How much more is hard to tell.

Since I noticed some tracks below the coolant pump weep hole it was on its way out and was going to need to be replaced sooner rather than later. So since Iím here anyway Iím going to replace the coolant pump, thermostat, pressure cap, and also the impeller cover was showing wear so Iíll replace that. Iíll put a new belt on while Iím at it.

I also picked up a set of end plate gaskets/o-rings, that will let me eyeball the tube bundle.

I was going to have to learn this stuff anyway, no time like the present.
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Old 19-05-2021, 10:56   #50
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Re: Overheat at low rpm - 3JH4E on Hunter 38

I assume the pump w/the weep hole is the raw water (sea water) pump and not the closed system coolant pump.

If you are seeing water out of the weep hole in the raw water pump it probably needs a major rebuild. If you buy a new one you could keep the old one and try to rebuild it for a spare. It most likely be a whole pump w/a new plate and impeller ready to go.
For simplicity would change the cap and belt, then give it a try. While thermostats do go bad, it isn't that common (I haven't seen it too much)
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Old 19-05-2021, 11:50   #51
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Re: Overheat at low rpm - 3JH4E on Hunter 38

Quote:
Originally Posted by flightlead404 View Post
Prop is clean

Qualitatively ďmoreĒ water exits at 2,000. How much more is hard to tell.

Since I noticed some tracks below the coolant pump weep hole it was on its way out and was going to need to be replaced sooner rather than later. So since Iím here anyway Iím going to replace the coolant pump, thermostat, pressure cap, and also the impeller cover was showing wear so Iíll replace that. Iíll put a new belt on while Iím at it.

I also picked up a set of end plate gaskets/o-rings, that will let me eyeball the tube bundle.

I was going to have to learn this stuff anyway, no time like the present.
Sounds like the best plan for you to take, but the exhaust water looks to be adequate, but only a mechanic knowledgeable on your exact engine would be abe to verify this.

Best of luck,

Wayne
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Old 19-05-2021, 15:33   #52
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Re: Overheat at low rpm - 3JH4E on Hunter 38

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I assume the pump w/the weep hole is the raw water (sea water) pump and not the closed system coolant pump.
No the fresh water/coolant pump
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Old 19-05-2021, 16:01   #53
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Re: Overheat at low rpm - 3JH4E on Hunter 38

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Just for giggles I decided to put everything back and run the engine expecting the engine driven pump to have a little more oomph than the bilge pump.

I *feel like* thereís more water out the exhaust. But no quantitative measurement and, as I mentioned before, not enough experience to know if its the appropriate amount of water (or just me being hopeful). I took some video at idle and in neutral at 2,000 rpm which Iíll figure out how to link here.

I took some temperature measurements after running the engine in neutral at 2,000 rpm for 10-15 minutes.

Incoming sea water: 24 c
Top of heat exchanger: 63 c
Top of pressure cap: 69 c
Body of elbow: 34 c
Bronze water injection fitting to main elbow: 35 c
Heat exchanger end cap (coolant out fitting): 39 c
Coolant pump body: 68 c
Top of thermostat: 75 c

Here is a video of the exhaust at idle

Here is a video of the exhaust in neutral at 2,000
IMO, the water flow at 2000rpm is low. I would have expected a more noticeable increase in water flow at 2000 - but it is subjective and I might be wrong.

The temperature numbers taken in neutral are meaningless (unless they were very high) as the engine is not developing any real power at 2000rpm in neutral.

Heat is produced only when power is being developed i.e. under load. You could try the test while tied to the dock in gear and get a more meaningful set of numbers. The engine should run under load at WOT without overheating.

Good hunting.
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Old 20-05-2021, 08:30   #54
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Re: Overheat at low rpm - 3JH4E on Hunter 38

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IMO, the water flow at 2000rpm is low. I would have expected a more noticeable increase in water flow at 2000 - but it is subjective and I might be wrong.

The temperature numbers taken in neutral are meaningless (unless they were very high) as the engine is not developing any real power at 2000rpm in neutral.

Heat is produced only when power is being developed i.e. under load. You could try the test while tied to the dock in gear and get a more meaningful set of numbers. The engine should run under load at WOT without overheating.

Good hunting.
To clarify, are you suggesting that with the boat tied up in the slip, with a couple of good aft spring lines as part of that, I should run it in gear at 2k rpm for a while?
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Old 20-05-2021, 08:34   #55
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Re: Overheat at low rpm - 3JH4E on Hunter 38

New coolant pump, thermostat etc installed. However a coolant drain petcock sheared off so I can’t put it back together and run it until tomorrow afternoon.

Also, I can’t get the end caps off the heat exchanger. The socket head bolts just won’t budge and I’m close to rounding them off. So I’ll have to leave that for now.

Will test under load tomorrow afternoon.

The other thing I was thinking off was to run the BB flush again but this time raise the bucket above the level of the heat exchanger hoping to ensure its fully flooded internally.
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Old 20-05-2021, 08:43   #56
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Re: Overheat at low rpm - 3JH4E on Hunter 38

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To clarify, are you suggesting that with the boat tied up in the slip, with a couple of good aft spring lines as part of that, I should run it in gear at 2k rpm for a while?

Short answer is yes unless you want to take it out for a spin. If you do it at the dock, tie/secure it well and slowly bring up the rpm. Check your lines/cleats in the process to make certain everything can take the strain. Always nice to have someone up top watching when you are going below to check the motor.
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Old 20-05-2021, 09:06   #57
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Re: Overheat at low rpm - 3JH4E on Hunter 38

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Also, I can’t get the end caps off the heat exchanger. The socket head bolts just won’t budge and I’m close to rounding them off. So I’ll have to leave that for now.
Don't take them off!!!! I did last year and the resulted in taking the whole thing apart and a machine shop visit.

From an earlier post:

Incoming sea water: 24 c
Top of heat exchanger: 63 c
Top of pressure cap: 69 c
Body of elbow: 34 c
Bronze water injection fitting to main elbow: 35 c
Heat exchanger end cap (coolant out fitting): 39 c
Coolant pump body: 68 c
Top of thermostat: 75 c

None of these seem hot really. I have not read the whole thread, but are you sure your sensor has just not gone bad?
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Old 20-05-2021, 12:59   #58
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Re: Overheat at low rpm - 3JH4E on Hunter 38

Your thermostat is at the top of normal temp with no load. Take the boat out with someone so you can shoot temps. Replacing the thermostat, and SW pump is a great idea. The cooling side seems okay.
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Old 21-05-2021, 07:21   #59
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Re: Overheat at low rpm - 3JH4E on Hunter 38

Just a thought:

The heat sensor or the wire connected to it may have gone south. Make sure vibrations at higher rpms are not wiggling (sp) the wire loose causing a false indication. There are a number of threads re: checking heat sensors -- a google search, "how to check a heat sensor on yanmar boat engine" could be helpful.

-Rob
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Old 21-05-2021, 08:37   #60
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Re: Overheat at low rpm - 3JH4E on Hunter 38

A heat exchanger can almost always use a good cleaning. You may have room to rod it out in place by removing the end caps, with sandpaper wrapped around a dowel or coat hanger or try pulling it thru on a string. Do each tube. Tank Water cap can be suspect, as can the thermostat. Iíve done the dinghy pump thing, which can work if an obstruction is loose like a plastic bag, leaves, etc., but if barnacles...
Any radiator shop can boil it out, though itís the typical cramped engine job to get it out.
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