Does anyone replace thermostat...

[skipperpete]

Originally Posted by redneckrob

What exactly is the failure mode on a bimetal strip connected to a spring and plunger? I'm sure one has failed somewhere sometime but it has to be vanishingly unlikely. Far more likely that it's jammed with crud, which wouldn't require replacement, and far far more likely as several have pointed out that the hoses need to be preventively replaced. Proactively replacing a thermostat annually as it sounds like one engine manufacturer recommends is just lunacy!

Actually thermostat failure across the whole range of internal combustion engines is fairly high and very damaging. When most modern wax pellet type thermostats fail it is in the unsafe position i.e. , closed, and usually caused by the wax leaking past the push rod seal. If an engine has an overheat that is not related to the thermostat, it's possible that the wax pellet can reach a temperature way beyond it's safe survival range so it might be worth replacing it if you have , say , a loss of raw water flow resulting in a coolant temp over 100°C
Some engines are fitted with multiple thermostats in parallel to avoid the almost instant catastrophic overheat associated with the failure of a solo thermostat
In the good old days thermostats were of a fail-safe design, a copper bellows with a bit of volatile liquid in a negative pressure. When they got hot the bellows would expand and if the bellows failed the liquid and ability to remain pressurized was lost and the bellows would stay open.... problem was they failed a lot.

[a64pilot]

Originally Posted by redneckrob

What exactly is the failure mode on a bimetal strip connected to a spring and plunger? I'm sure one has failed somewhere sometime but it has to be vanishingly unlikely. Far more likely that it's jammed with crud, which wouldn't require replacement, and far far more likely as several have pointed out that the hoses need to be preventively replaced. Proactively replacing a thermostat annually as it sounds like one engine manufacturer recommends is just lunacy!

Never seen a bi-metallic motor thermostat, now the old ones that hung on the wall, yes.
No, I wouldn’t replace yearly either. However I replaced mine when I got the boat and flushed the coolant, cleaned the heat ex etc. not knowing if it were original or not, I replaced it and put the old one in spares.
Within the week I’m flushing and going with new coolant again, and will replace all the hoses this time. I don’t plan on replacing the thermostat even though it’s a few years old, but it only has 1000 hours too.
They are very reliable, one in our Prius is 10 yrs old and has 250,000 miles on it.
I assume the Prius has a thermostat, but thinking about it, it might not.

My Duramax Diesel has two, a small and a large on, the small one opened at a lower temp than the large on. A big issue with Diesels is that at idle they don’t make much heat and it may be that on the Duramax the large on stayed closed at idle.

The oil thermostat on my IO-540 Lycoming worked in reverse to an automotive thermostat, when cold, oil flow bypassed the oil cooler and flowed through it, when hot the thermostat closed and forced all oil through the cooler.
So if and when it failed what happened was all oil went through the cooler, meaning of course that you had an low oil temp problem to fix and not one that was too hot.
Auto engines I guess never figured this out and I guess an overheat just means you pull over, so they don’t have fail safe thermostats.

[Wotname]

Originally Posted by CassidyNZ

......

To come back on topic, I can’t recall ever replacing a thermostat that wasn’t faulty. Thermostats always fail to open when faulty - my fallback then is to remove it and get a new one when I next can. It’s really no biggie.

There is always an exception to the rule .

Here is a thermostat that failed in a partially open position and without a temperature gauge, there was no external indication of a problem.

http://www.cruisersforum.com/forums/...rt-149469.html

This experience promoted me to fit a temperature gauge however a say yearly test of the thermostat would have been anther option.

[CassidyNZ]

Originally Posted by Wotname

There is always an exception to the rule .

Yes I suppose there is but the exception will not (as in your case) present the engine with a desperate problem unless I guess, the half open attitude leads too insufficient coolant flow in which case, revert to my original comment

[charliehows]

Originally Posted by Wotname

Hmm... I don't see anything to worry about - assuming the waterways in the manifold are mainly clear of crud. There is no substantial pressure in the raw water cooling circuit, it is essentially open to the atmosphere or at least open to the back pressure in the exhaust system.

When the thermostat is fitted and the thermostat is in the cold position, all cooling water passes directly from the pump to the exhaust injection point via the bypass hose and none though the block/head/manifold.

When the thermostat is fully hot, all cooling water passes though the block/head/manifold and none though the bypass hose.

Your tap (when closed) in the bypass hoses is just replicating a fully hot thermostat. IMO, the tap (or any permanent blockage) is essential if you remove the thermostat.

actually when the t.stat opens on my manifold fitting it doesnt block the by-pass flow, it merely impedes it by intruding the sprung mechanism into the space normally open to the by-pass. Which is why I dont take the whole system very seriously. Also why the manifold is prone to blockage.

[alctel]

Interestingly, if you have a 2QM20 like mine it was built as a raw water engine and then at the factory they cludged on a coolant system.

However they didn't switch over the thermostat, so it still opens at 41c rather than at 72c like the fresh water thermostats

Doesn't seem to make any difference in running the engine however

[Compass790]

Originally Posted by alctel

Interestingly, if you have a 2QM20 like mine it was built as a raw water engine and then at the factory they cludged on a coolant system.

However they didn't switch over the thermostat, so it still opens at 41c rather than at 72c like the fresh water thermostats

Doesn't seem to make any difference in running the engine however

I don't own a 2QM20 but according to the parts list they came from Yanmar raw water cooled. If yours has a heat exchanger it is an aftermarket job.
Do you mean the factory that built your boat & installed engine?
Both Wotname & charliehows own 2QM20's so they'll chime in if I am wrong.

[Wotname]

The 2QM20Y and 2QM20F were coolant cooled from Yanmar as was the 3QM30Y and 3QM30F.

The 2QM20, 2QM20B, 2QM20H, 3QM30 and 3QM30H were raw water cooled.

Detail is everything

[a64pilot]

Originally Posted by alctel

Interestingly, if you have a 2QM20 like mine it was built as a raw water engine and then at the factory they cludged on a coolant system.

However they didn't switch over the thermostat, so it still opens at 41c rather than at 72c like the fresh water thermostats

Doesn't seem to make any difference in running the engine however

I’m not surprised they didn’t change thermostat temp, the engine should have been designed so that the clearances were correct for a certain temp, change temps and you change clearances.

[alctel]

Originally Posted by Compass790

I don't own a 2QM20 but according to the parts list they came from Yanmar raw water cooled. If yours has a heat exchanger it is an aftermarket job.
Do you mean the factory that built your boat & installed engine?
Both Wotname & charliehows own 2QM20's so they'll chime in if I am wrong.

Sorry, I missed a comma! They made a raw water and a fresh water version of the 2QM20, for a large batch of the raw water ones (2QM20H) for whatever reason they decided to convert them to fresh water at the factory, and bolted on a third party heat exchanger. I'm pretty sure it was done at the yanmar factory as a bunch of boats of different makes had the same thing. It's not the 2QM20F, which has the 'proper' freshwater setup. They didn't bother switching over the thermostat for these ones, so it still has the raw water thermostat and the thermostat for the 2QM20F doesn't fit, so you can't switch them.

Still a great engine (despite being massive and weighing a huge amount for 20HP)

[Compass790]

Originally Posted by alctel

Sorry, I missed a comma! They made a raw water and a fresh water version of the 2QM20, for a large batch of the raw water ones (2QM20H) for whatever reason they decided to convert them to fresh water at the factory, and bolted on a third party heat exchanger. I'm pretty sure it was done at the yanmar factory as a bunch of boats of different makes had the same thing. It's not the 2QM20F, which has the 'proper' freshwater setup. They didn't bother switching over the thermostat for these ones, so it still has the raw water thermostat and the thermostat for the 2QM20F doesn't fit, so you can't switch them.

Still a great engine (despite being massive and weighing a huge amount for 20HP)

Thanks for schooling us up. That's interesting. I'd be looking for a thermostat at the local auto supply place that fits or could be modified to fit. Not that it would make much difference but if you have a heat exchanger it seems a waste not to utilise the benefits of a higher temp thermostat.

[charliehows]

Originally Posted by alctel

They didn't bother switching over the thermostat for these ones, so it still has the raw water thermostat and the thermostat for the 2QM20F doesn't fit, so you can't switch them.

Still a great engine (despite being massive and weighing a huge amount for 20HP)

They probably didnt 'bother' to switch the thermostat because they didnt 'bother' to redesign the engine to run at a much higher temperature. One of the main functions of a heat exchanger is to allow engines designed to run in surface vehicles to be used in seawater cooled vehicles without redesigning them to run at much lower temperatures to avoid raw seawater salt precipitation.

[Compass790]

Originally Posted by charliehows

They probably didnt 'bother' to switch the thermostat because they didnt 'bother' to redesign the engine to run at a much higher temperature. One of the main functions of a heat exchanger is to allow engines designed to run in surface vehicles to be used in seawater cooled vehicles without redesigning them to run at much lower temperatures to avoid raw seawater salt precipitation.

Hmmm???
I'd put my dinner on the fact that they haven't "redesigned" the 2QM20F to allow the cooling temp difference from the 2QM20.
The only differences would be around the cooling system.The engine design would be unchanged.I'd be stunned if you can prove me wrong on that but I've been stunned before.

[Wotname]

Should anyone be interested...

Yanmar QM series raw water cooled have a thermostat rated 42/52 C. This thermostat is mounted in the exhaust manifold.

Yanmar QM series coolant cooled have a thermostat rated 71/85 C. This thermostat is mounted in the coolant tank.

There is no difference in part numbers for the pistons/rings/bearing/valve guides etc.

If you have a coolant cooled engine with the thermostat in the exhaust manifold - it isn't factory but perhaps distributors/others have made the alterations.

[charliehows]

I think it may be fair to conclude that the same engine is capable of operating in both temperature ranges - ie 50ishC raw water or 80ishC fresh water - which is why I'm pretty cavalier about chucking out the thermostat - I have made sure the temperature gauge is working, which is a much better way to make sure the engine is operating in a safe range (already had a good test of that when one of the internal anodes came loose and blocked the waterway - she ran into overheat within 10 minutes of starting.)