Yanmar Engines

[RustyBucket]

I have a Yanmar 2QM20H engine with fresh water cooling. The manual calls for a thermostat that is around 160 degree top temperature. The part number in catalog calls for a thermostat that is physically incompatible with the engine set up. The thermostat for the sea water cooling does fit perfectly but runs at around 100 degrees. I want to replace my current thermostat.Any suggerstion?

[Gary Mc]

Thermostats are pretty generic, I would think you could find what you need at any auto parts store?

[Compass790]

Originally Posted by RustyBucket

I have a Yanmar 2QM20H engine with fresh water cooling. The manual calls for a thermostat that is around 160 degree top temperature. The part number in catalog calls for a thermostat that is physically incompatible with the engine set up. The thermostat for the sea water cooling does fit perfectly but runs at around 100 degrees. I want to replace my current thermostat.Any suggerstion?

That must be a conversion because AFAIK that H engine came from the factory as raw water cooled. Can you contact the owner that did the conversion?
Are you sure its not a fresh water cooled 2QM20Y (F)? That has a thermostat with 185F top temp. I'd check out a 2QM20Y (f) thermostat to see if it fits.

[bigdogandy]

Maybe a bit off topic, but what’s the harm in just taking the thermostat out entirely?

[RustyBucket]

Originally Posted by bigdogandy

Maybe a bit off topic, but what’s the harm in just taking the thermostat out entirely?

Very very bad idea. You have no control over cooling system

[RustyBucket]

Originally Posted by Compass790

That must be a conversion because AFAIK that H engine came from the factory as raw water cooled. Can you contact the owner that did the conversion?
Are you sure its not a fresh water cooled 2QM20Y (F)? That has a thermostat with 185F top temp. I'd check out a 2QM20Y (f) thermostat to see if it fits.

The conversion came from factory and previous owner has no clue. Trying a thermostat for F or Y version not bad idea.

[Wotname]

Originally Posted by Compass790

That must be a conversion because AFAIK that H engine came from the factory as raw water cooled. Can you contact the owner that did the conversion?
Are you sure its not a fresh water cooled 2QM20Y (F)? That has a thermostat with 185F top temp. I'd check out a 2QM20Y (f) thermostat to see if it fits.

Wot he said plus 1.

[PerfectPirate]

I do not know which manual you are looking in, but the Yanmar Service Manual for the 2QM20 specifies that in the raw-water cooled version (2QM20(H)), with a 2.2:1 reduction drive, the thermostat will begin to open at a temperature of 42 C (108 F) and be fully open at a temperature of 52 C (126 F). In 1975, the Yanmar part number for that thermostat was 124770-49200, but it may have been revised since.

In the fresh-water cooled version (2QM20Y(F)), with a tank, heat exchanger and recirculating coolant, the manual specifies that the thermostat will begin to open at a temperature of 71 C (160 F) and be fully open at a temperature of 85 C (185 F). I do not have a part number for that type of thermostat, but I do know that it is supposed to be located on the fresh water tank (heat exchanger), at the point where the water flows into the tank from the cylinder head. Whereas on a raw-water cooled engine, the thermostat is housed on top of the exhaust manifold, linking into the cooling water bypass circuit.

So you appear to have one model of engine (raw-water cooled) while quoting the thermostat specs for a different engine (fresh-water cooled). It would appear that Compass790 has put his finger square on the root of the problem, that you are stuck with a conversion. Which then leads to other questions: Do you in fact have a fresh water conversion and is there a thermostat on the fresh water tank? If so, then the manual seems to indicate that you should not have any thermostat on top of the exhaust manifold, but rather the that the raw sea-water should be pumped directly into the heat exchanger and then flow directly from the exchanger into the exhaust manifold, without passing through a thermostat. Only the fresh water loop should be passing through a thermostat, the one on the inflow into the tank.


Let us know how it all works out.

Originally Posted by bigdogandy

Maybe a bit off topic, but what’s the harm in just taking the thermostat out entirely?

Andy, the thermostat is necessary to ensure a proper operating temperature for the engine. A diesel engine needs to be warmed up to a specific temperature range in order to run and properly burn fuel, which is why the engines in trucks and construction gear have glow plugs to preheat the cylinders. If you simply do a full-flow of cooling water from the get-go, then the engine will have difficulty warming up enough to facilitate full combustion, especially in a cooler climate.

[RustyBucket]

The manual that came with the boat says it’s a 2QM20H.
The conversion to fresh waterways done by manufacturer.
The thermostat is on the manifold and the fresh water tank is a cylinder but has no thermostat.
The part number fro a Yanmar parts catalog says that the fresh water thermostat number ends with 800 whereas the raw water one ends with 200.
Physically the raw water one fits but the 800 one can’t fit.
The parts manual confirmed the higher temperature . Since I am currently measuring temperature on top of manifold I would expect to read the higher numbers.
What to do?

[RustyBucket]

I am currently using a generic thermostat and get a temperature reading around 140 at 1500 rpm steady

[PerfectPirate]

Hey Rusty, the 800 one would not be able to fit because it is not designed to go into the top of the exhaust manifold, but into the side of a Yanmar fresh water tank. However, the tank illustrated in the service manual is not cylindrical, but rather rectangular in shape. If Yanmar, the manufacturer, actually did the conversion then I would have expected them to provide a thermostat control on the freshwater cooling loop. If I properly understand your response, you are currently using the generic thermostat on the top of the exhaust manifold, where the seawater exiting the heat exchanger enters the exhaust manifold.

The possible problem with this is that this thermostat is not actually responding to the temperature of the coolant circulating through the engine, but rather the raw water passing through the exchanger. It is a far less precise control over the temperature of the actual coolant and the raw water exiting the exchanger will be at a much lower temperature than the actual coolant. That said, 140 F seems to be a fairly reasonable operating temperature for the engine. My 2QM20 is raw-water cooled and only has an overheat idiot light rather than an actual temperature gauge. I have not yet had the opportunity to monitor its actual temperatures with a heat sensing gun, but I look forward to being able to do so.

So, are you intensely unhappy about the 140 degrees?

[Wotname]

Originally Posted by RustyBucket

The manual that came with the boat says it’s a 2QM20H.
The conversion to fresh waterways done by manufacturer.
The thermostat is on the manifold and the fresh water tank is a cylinder but has no thermostat.
The part number fro a Yanmar parts catalog says that the fresh water thermostat number ends with 800 whereas the raw water one ends with 200.
Physically the raw water one fits but the 800 one can’t fit.
The parts manual confirmed the higher temperature . Since I am currently measuring temperature on top of manifold I would expect to read the higher numbers.
What to do?

What is the model number as marked on the engine identifier plate which is located on the side of the block behind (underneath) the starter motor? Whatever is there is how Yanmar built the engine.

A freshwater conversion was made by Yanmar using Yanmar parts and (AFAIK) also by other third parties mostly using Yanmar parts. However not everyone managed to fit these conversions properly. It really sounds like you have one that hasn't been properly converted.

As Perfect Pirate states, you can't successfully control the coolant temperature by using a raw water thermostat.

Remember the raw water circuit should never be allowed to exceed ~50-55 C as doing so rapidly increases calcium carbonate (boiler scale) in the exhaust manifold.

[RustyBucket]

I am feasibly happy with my current temperature controls but wonder why the factory installed a fresh water system but did not follow their own design. As I look at my manual again I see the design for the salt water configuration and the next section shows a fresh water configuration with the thermostat into the fresh water intake side of the heat exchanger. Here is the rub... that configuration is for the f and y models and not the H. The manual does not mention the H model with fresh water. They left the thermostat on the exhaust manifold which explains why the two thermostats are physically different and operate at different temperatures.
My setup actually reads the temperature of fresh water as it passed either into the engine or bypasses it and goes into fresh water tank.
It seems that tts some hybrid solution to an evolving model change. It explains why the specs call for a different thermostat than the one that fits the H model.
My fresh water tank has no opening for a thrmistat. Thanks for opening my eyes

[Wotname]

Originally Posted by RustyBucket

I am feasibly happy with my current temperature controls but wonder why the factory installed a fresh water system but did not follow their own design. As I look at my manual again I see the design for the salt water configuration and the next section shows a fresh water configuration with the thermostat into the fresh water intake side of the heat exchanger. Here is the rub... that configuration is for the f and y models and not the H. The manual does not mention the H model with fresh water. They left the thermostat on the exhaust manifold which explains why the two thermostats are physically different and operate at different temperatures.
My setup actually reads the temperature of fresh water as it passed either into the engine or bypasses it and goes into fresh water tank.
It seems that tts some hybrid solution to an evolving model change. It explains why the specs call for a different thermostat than the one that fits the H model.
My fresh water tank has no opening for a thrmistat. Thanks for opening my eyes

I very much doubt a 2QM20H ever left the 'factory' with a coolant cooling system.

It is possible that it left a Yanmar distributor or a Yanmar agent with coolant cooling or just as likely, it was added by a subsequent owner. The last 2QM20(x) left the factory ~41 years ago and much could have happened to it in that time period.

Depending on location and prevailing trade conditions, Yanmar would ship engines from the manufacturing plant only ~95% assembled. The country distributor would add the final components before delivery to the customer thus allowing it to be marketed as 'assembled in (insert country of choice)'. Cooling components were typically added at this time.

[Benz]

Slight thread drift: does it annoy anyone else that Yanmar prints manuals for 3 different models as one book? With all the engineering and spec differences between a 1gm and 3gm, why not print a manual for each? It's well-nigh impossible to decipher the triple manual, and trying to figure out which schematic you're looking at is not easy, since they don't label them clearly.