I have a 3QM30 (Raw Water Cooled) Yanmar which has 2 thermostats installed, but the local Yanmar mechanic and service manual drawings both indicate I should only have one installed. The housing is built to take 2 thermostats (in parallel) and if I remove one, then the raw water will bypass the remaining thermostat rendering it ineffective. The motor runs fine (temperature wise) with 2 thermostats installed.
Have any of you come across this " 2 Thermostat" conundrum previously. I googled it and found a few references to the same issue but nothing official.
My thoughts are that I will be using 2 thermostats and dare I say, "Yanmar may not have delved deep enough into my request for information and ......." The local mechanism did his best to find out, but he is also relying on Yanmar to give him the correct information as well.
(The thermostat housing is built into the exhaust manifold and is different from the 2QM20.)
Cheers 
Select to expand quoteCrusoe said..
I have a 3QM30 (Raw Water Cooled) Yanmar which has 2 thermostats installed, but the local Yanmar mechanic and service manual drawings both indicate I should only have one installed. The housing is built to take 2 thermostats (in parallel) and if I remove one, then the raw water will bypass the remaining thermostat rendering it ineffective. The motor runs fine (temperature wise) with 2 thermostats installed.
Have any of you come across this " 2 Thermostat" conundrum previously. I googled it and found a few references to the same issue but nothing official.
My thoughts are that I will be using 2 thermostats and dare I say, "Yanmar may not have delved deep enough into my request for information and ......." The local mechanism did his best to find out, but he is also relying on Yanmar to give him the correct information as well.
(The thermostat housing is built into the exhaust manifold and is different from the 2QM20.)
Cheers
Have no drawing but to me its . knowing you have a raw water cooling system this engione may have been also built to run coolant in a closed circuit and the raw sea water cools the coolant via a heat transferrer unit ( cant thing of the correct name right now)
I may have been put there for an optional closed circuit so the raw water cooling the engine coolant .
One for the engine cooling system and one for the the raw sea water all depending on what the drawing looked like and if the cooling circuit proves that.
Would depend on how the thermostat housing appeared meaning two systems one to reduce the raw water flow and one for the engine coolant.
That all a shot in the dark and a little bit of creative writing 
One other thing comes to mind perhaps your engine was made. For cold climate near the north or South poles
Are the two thermostats the same opening temps or are they different to each other
Do you think there is any other coolant path
ways either could take?
The other think perhaps there might be a therostat that opens to direct warm water to a hot water system opens when it get warn
Or an oil cooler
Good vids Cisco. I agree with your praise for 2QM15, I have just inherited one that has not run for 5 years - still in boat (cramped access) and clearly in need of serious attention. The mixer elbow has corroded through to the extent that water was being discharged into the engine bay (and rest of the bilge). Apart from replacing mixer pipe I will need to remove and investigate damage to exhaust manifold and possibly the head. My question is, where does one source parts for these engines in Australia, have found one European site whose prices are astronomical, but, know not of a supplier/s here in S.A., or Australia for that matter. CAn you shed any light?
Cheers, Nautique.
Select to expand quoteNautique said..
Good vids Cisco. I agree with your praise for 2QM15, I have just inherited one that has not run for 5 years - still in boat (cramped access) and clearly in need of serious attention. The mixer elbow has corroded through to the extent that water was being discharged into the engine bay (and rest of the bilge). Apart from replacing mixer pipe I will need to remove and investigate damage to exhaust manifold and possibly the head. My question is, where does one source parts for these engines in Australia, have found one European site whose prices are astronomical, but, know not of a supplier/s here in S.A., or Australia for that matter. CAn you shed any light?
Cheers, Nautique.
I'm not sure but perhaps some of the small Yanmar excavators may be a source, theres a few wreckers around ( I think there was one in Bendigo). I guess they wont have marine manifold but other spare parts might be had second hand from these sources
Select to expand quoteNautique said..
My question is, where does one source parts for these engines in Australia, have found one European site whose prices are astronomical, but, know not of a supplier/s here in S.A., or Australia for that matter. CAn you shed any light?
Cheers, Nautique.
www.minardsdiesel.com/
Minards in Newcastle - they have, or can get almost anything you need. I purchased an exhaust mixing elbow, plus other, from them no fuss.
Select to expand quoteHG02 said..
One other thing comes to mind perhaps your engine was made. For cold climate near the north or South poles
Are the two thermostats the same opening temps or are they different to each other
Do you think there is any other coolant path
ways either could take?
The other think perhaps there might be a therostat that opens to direct warm water to a hot water system opens when it get warn
Or an oil cooler
Thanks HG02, There appears to be only one coolant path setup unless some modifications were to done. The water from the raw water pump splits in 2 and one path goes straight to the exhaust manifold where it cools it and the other path is into the engine block and through the head. The water flow in this second path is limited by the thermostat just before the coolant can also enter the exhaust manifold.
With regard to cooler climates/water, less water through the engine would be better to allow it to reach operating temperature, meaning only one thermostat would be required.
There are no other coolant path outlet/connections for a hot water system. The housing for the 2 thermostats has them in parallel.
Thanks Lazzarae - had bumped into their website at one time, they didn't seem to have on-line catalogue or ordering system - I will look again and also note phone contact detail. Nautique.
Select to expand quoteNautique said..
Good vids Cisco. I agree with your praise for 2QM15, I have just inherited one that has not run for 5 years - still in boat (cramped access) and clearly in need of serious attention. The mixer elbow has corroded through to the extent that water was being discharged into the engine bay (and rest of the bilge). Apart from replacing mixer pipe I will need to remove and investigate damage to exhaust manifold and possibly the head. My question is, where does one source parts for these engines in Australia, have found one European site whose prices are astronomical, but, know not of a supplier/s here in S.A., or Australia for that matter. CAn you shed any light?
Cheers, Nautique.
Our local marine mechanic has no trouble sourcing parts for my engine. Don't know where he gets them but his prices seem reasonable.
Re your mixing elbow. genuine Yanmar elbows are made of cast iron, complicated and seem to chew out fairly quickly. Instead of genuine I would get one fabricated from s/steel. Just a quarter or three sixteenth of an inch plate shaped and drilled to fit the manifold with a straight piece of pipe welded into it and then the half inch water injection pipe welded into that so it projects down the exhaust pipe by two or three inches and positioned and angled so that you can use a length of standard heater hose between it and the manifold.
Hope this helps.
Thanks Cisco for continuing to give the grey matter a work over with regard to my problem. I had though of this also but the parts manual only has one type of thermostat listed. I also thought the thermostat was the same as the one used in the 2QM20 and therefore require 2 to give the volume of flow required for correct cooling, but they are listed as different thermostats/part-numbers.
I have just been looking through the parts manual again and noticed a picture of a different type of cover that can be used. It appears to only have one coolant connection point where as the one I am using has 2. Maybe this second type of cover is capable of utilising only one thermostat. Anyway, logic dictates my existing coolant path requires the second location for a thermostat be blocked off to get the first thermostat to operate correctly. As I don't have a suitable plug I will be using the second thermostat to block the orifice. I can't see a problem using 2 as they will just adjust their flow to suit the temperature. They may hunt a bit but this may help to keep them from seizing in position as I have seen previously with other thermostats.
The second type of cover may even be for a Hot Water system as HG02 suggested
Thanks for your help,
That may be of help Cisco. The mixer pipe does seem to be a problematic area, a design flaw perhaps. After watching a Youtube vid of one removed for replacement and fixing 'downstream' probs. such as water backwashed into head and cylinders I picked up that the author had sourced a S/Steel mixer pipe from N.Z. for about $200 - I think I have a link to that supplier somewhere - that would seem v.reasonable (price) if not able to get one fabricated here from S/Steel or too costly to do same. But a good idea nonetheless. Tks Cisco. Nautique.
Also look at the two thermostats as a bonus as if one fails to open the other one will open further and still control the temp
I have a 3QM30 and run mine with two thermostats installed. The engine runs fine with this arrangement. I thought the reason for the two was that the 3QM30 is a rather large engine for its 33hp rating with large raw water galleries. My reasoning is that in having the two thermostats allows for better water flow when open. The two thermostats are the same part number. I replaced mine a few years ago and sourced them from Minards Marine at Newcastle.
