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Old 11-10-2017, 06:48   #1
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Yanmar Fresh Water Thermostat

My new old sailboat Sequoyah has a 1999 Yanmar 3GM30F with a Y/M thermostat no. 121750-49800 in the fresh water cooling system. It's old, and I suspect tired, so in a fresh (so to speak) start I'd like to replace it.

The genuine Yanmar part is the price of a good dinner out, and if possible I'd rather eat well than stick needlessly with OEM parts. Does anyone know of a more reasonably priced substitute for this thermostat?
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Old 12-10-2017, 03:37   #2
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Re: Yanmar Fresh Water Thermostat

Thanks for considering the question. With 73 readings and no answers as of this morning, I'll take it as a "No, you have to get off it and buy the OEM part. Just have a salad instead of a steak when you go out."
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Old 12-10-2017, 04:47   #3
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Re: Yanmar Fresh Water Thermostat

Well if you're willing to take a chance, measure yours and have a look at this Dayco page (there are 4 pages).

Dayco - Thermostats


You might find substantial savings, or maybe none at all...

If you find the right one, at a good price, I know there are several people on this forum who'd love to hear about it...
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Old 12-10-2017, 07:38   #4
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Re: Yanmar Fresh Water Thermostat

I'll be digging (well, unbolting, prying off, etc.) into this next week when I go back aboard. I indeed will measure, compare, check opening temperatures, and report back. Thanks for the link.
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Old 12-10-2017, 09:55   #5
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Yanmar Fresh Water Thermostat

I am certain that is is a common thermostat, just like roller bearings are.
Trick is of course finding the right one. Mine is a dual opening thermostat, yours may be too.
People often buy into the theory that OEM oil filters etc. are superior products and worth a premium, when almost always that is not the case.
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Old 12-10-2017, 10:13   #6
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Re: Yanmar Fresh Water Thermostat

I'm hoping you're right. I rebuilt the water pump">raw water pump a few weeks ago, specifying parts from the old ones once it was dismantled. Two sealed metric ball bearings, one spring-tensioned shaft seal, a metric O-ring for the cover plate, and a second small O-ring to serve as a slinger did the job for under twenty bucks, not counting the impeller (went with a more costly "run dry" one for that). Looking at this thermostat it's hard to imagine it fetching more than ten in a competitive market, so it's all a matter of specing it out and finding a generic.

Or so it ought to be. We'll see. As everyone knows, you cain't always git whut you want. I'll post here once I've tried to.
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Old 12-10-2017, 10:24   #7
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Re: Yanmar Fresh Water Thermostat

A trip to the right auto parts store and you may be able to find one that fits. But realistically, put it in a pan of water, heat it up and if it opens, keep using it. The default plan if you ever have trouble is jut to remove it until you find one. It's not an "I'm stuck here and cant move my boat" type of item.
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Old 12-10-2017, 10:46   #8
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Yanmar Fresh Water Thermostat

Keep the old one as a spare, if its a dual valve thermostat, simply removing the thermostat will not allow the engine to cool properly like it will with a single valve thermostat.
Reason is the disk at the bottom of a dual valve closes off the bypass circuit as the regular circuit is opened up, it’s a more modern solution that I believe more tightly controls operating temperature.
My 30yr old 4JHE has a dual valve, I think they are common in Yanmars.
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Old 12-10-2017, 12:28   #9
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Re: Yanmar Fresh Water Thermostat

This is one of the long frame thermostats, discs top and bottom, so I think it must be a dual. I'll know more along about the end of next week. That's when I'll haul out the wrenches and get to work on it.
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Old 12-10-2017, 13:17   #10
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Re: Yanmar Fresh Water Thermostat

Why do you think it is bad. Is the engine overheating or not getting up to proper operating temp?

Take the thermostat, put it in a pot of water and turn on the heat. You can watch it open as the water heats up. It should be full open before the water boils - around 180-190 degrees Fahrenheit. Let it cool and watch it close.
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Old 12-10-2017, 13:42   #11
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Re: Yanmar Fresh Water Thermostat

That sounds like good advice. Thanks.

I think it may be bad because the motor is indeed overheating. I've been through about half the checkout of the raw water system and intend to complete that first, beginning with the mixing elbow "breathalyzer" test, then pull the fresh water thermostat. I'll try it as you suggest.
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Old 12-10-2017, 19:28   #12
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Re: Yanmar Fresh Water Thermostat

I suggest you poke around the parts sites and Yanmar handbooks to find specs on the item. Then look at a generic supplier like Stant. I found my Volvo-Penta thermostat for $5 vs $50 from VP.
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Old 13-10-2017, 03:10   #13
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Re: Yanmar Fresh Water Thermostat

Yanmar are downright stingy with specs. So far I've not located a third party to provide them, but I'll keep trying and will post them here once I have them, either from another source or from my own examination of the old one.
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Old 13-10-2017, 06:13   #14
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Re: Yanmar Fresh Water Thermostat

If you don't know when it was done, just replace that mixing elbow. Specs call for replacement every few years anyway, if the old cast iron ones.
You can buy aftermarket stainless steel ones for the same price
Other causes of over heating, added the wrong antifreeze causing it to gell and not circulate.
Clogged heat exchanger or exhaust elbow.
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Old 13-10-2017, 10:08   #15
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Re: Yanmar Fresh Water Thermostat

The mixing elbow is the "old style" Yanmar OEM, not the even older Christmas tree style but the smoothly curved one. It's not their newest version, which is boxy. The factory paint appears never to have been cracked on it.

Antifreeze looks good, but I'm thinking a flush is in order anyway, just to get a new baseline.

I took off the heat exchanger's forward end cap, where the raw water enters. There was a little sediment and a small piece of shell at the bottom, but above the lowest row of tubes it appeared quite clean. I intend to get a tube brush once I've measured the diameter of those and make sure, but I don't think that's an issue.
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