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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 
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.
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.
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.
