Thermostat Needed?

[Milton Bertin]

My sailboat comes with a Yanmar 3qm30, that starts to develops a heating problems at 2000 rpm loaded, I’m not near my boat now, but thanks to this forum I know that I needs to go to see the mixim elbow in my net trip.

But in the search that I do, I remove the thermostat (you know, a closed thermostat produce over heating) and I don’t put it again back, my understanding is that thermostats are there to make possible a shorter heating time of the motor, specially in cold weather. So as far as I’m traveling in the hot weather of the Panama my understanding is that I really don’t need the thermostat.

Any comments??

[svHyLyte]

A thermostat is designed to maintain a relatively constant temperature in the engine throughout its range of operation. Without the thermostat, and depending upon ambient conditions, the engine will either run too hot; or, too cold, each of which is damaging to the engine. (For more information see Thermostat - Wikipedia, the free encyclopedia ) It would be very unwise to remove the thermostat from one's engine.

[Ex-Calif]

Does the engine make maximum rated RPM in gear?

When our prop is getting fouled we experience overheating combined with inability to reach maximum RPM.

[Solitude]

You do need to keep a thermostat in. There are many things that can make an engine overheat. Removing the thermostat will only resolve the issue as a temporary measure. Your engine needs to run at the right regulated temperature. Look under engine overheating and check all the items one at a time. Some simple ones are..

restricted water intake
clogged heat exchanger
poor circulation
impeller problems

[mesquaukee]

You can check your thermostat by putting it in a pot of water and heating it. You will see it open. The temperature range is dependent on the application but should be somewhere around 160-170 F when it is fully open.

[Albro359]

I have a 3QM30F and I have had trouble maintaining temperature.
With a standard thermostat it overheats.
The reason I think is that the time constant for temperature change in my system is too long....

Why ?

I have a hot water cylinder plumbed into the system with about 6m of lagged pipe from the engine to the hot water cylinder heat exchanger and back again.

What I did to try to solve the problem was drill some small (3mm) holes in the rim of the thermostat to act as a bypass....6 seems to be the right number (about 70degC) ...4 it still runs hot and 8...or 6 x 4mm holes is too much and it runs cool...about 55 degC

I haven't found any information anywhere that tells me what temperature this engine should run at...does anyone know for sure ?

All the best

Alan

[GordMay]

Yanmar Thermostats are typically designed to open at 180 degrees F. (82 deg. C)
The 3QM30F engine uses a #124790-49800 Thermostat & #124790-49760 Gasket.

[Albro359]

Thanks Gord,

So that's the approx temp they should run at...~80degC ?

What harm can be done by running at too low a temperature ?

Cheers

Alan...

[NCboatrx]

Many of the QM series of engines were raw water cooled and their operation temp was about 140 degrees F. On these engines there is a tendancy for the passages in the exhaust manifold to block with carbon alongside the thermostat and this will cause overheating.

The freshwater cooled engines should run at about 180 degrees F.

The thermoststs and exhaust manifolds are completely different for the different cooling systems.
Stanley

[Hyprdrv]

Allen,
Most issues with low engine temps deal with the oil temperature remaining low and not circulating properly. If the oil is thick (cold) it doesn't circulate and lubricate as good as at the proper engine temperature. Also engines are built with a heated tolerance meaning that cold, the gap between the cylinder and piston may be .005" where hot the gap closes to lets say .001". That may not sound like much but in the world of mechanical things it can cause all kind of problems. It's important to maintain the manufactures' parameters. Also figure it this way, do you think the manufacturer would add a part that wasn't necessary?
When dealing with overheating issues it is often missed that the block has become "calcified" or (non tech term) "crudded up". This could be rust or a number of other "cruddy" items that restrict or inhibit the heat transfer to the cooling fluid. Lets say you got a 1/4" water passage in a block that has been reduced to 1/8" do to rust. First off you have reduced the water flow by 1/2 which could require twice the water flow to do the same job. Second you're trying to transfer the heat from the engine to the water and now have to go through an additional layer of crap. This could retain the heat in the block well above the operating temperature at the heat source since you've added another layer the heat has to transfer through. Old Adobe homes would absorb the sun's heat all day and keep the interior cool. At night when the temps dropped outside the heat would continue to transfer into the house warming the interior at night. Once the proper heat transfer coefficient was determined you could figure the thickness of the wall and maintain the temperature inside for 24 hours. With an engine the same thing happens, the rate of transfer is slower and the heat side of the block gets a lot hotter causing additional ware.

[Albro359]

Thanks Guys...mine's a 3QM30F...from new fresh water cooling....

Re the "crud" issue...I did a top end overhaul last year and the water galleries were remarkably free of crud (but its always been fresh water with coolant)...also the exhaust manifold was reamed out and the heat exchanger tubes cleaned...the water flow is good..

BTW if you reduce a hole from 1/4 to 1/8 you get only 1/4 the flow not 1/2...its the cross sectional are ...and that's radius squared so 1/2 the diameter and 1/4 the flow...but I take your point...

I'm sure the issue is the long lines to the hot water system ...and I do have a solution to it...just not very mathematically / engineeringly precise !

[GordMay]

Originally Posted by Albro359

...I'm sure the issue is the long lines to the hot water system ...and I do have a solution to it...just not very mathematically / engineeringly precise !

I cannot think why.

The Yanmar Help site may have something further to say on the subject (lots of basic info' there):
➥ Yanmar Marine Engine Help