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Old 02-06-2012, 14:30   #1
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Yanmar 3HM35F thermostat question

Can any of you experienced yanmar drivers tell me what the symptoms would be if the thermostat were stuck closed on a 3HM35 with raw water cooling system? I don't have temperature gauges with the engines, so can't read the operating temp or compare one to the other.

Can a stuck thermostat cause the engine to run roughly and put out white smoke? It's not a fuel issue, and the raw strainer and impeller look fine. this problem doesn't crop up until after the engine has been running for at least an hour, and we are in warm water


(I got two of these engines with the boat we just bought, and am slowly working my way through a number of issues. This is the latest. Hard shifting is another.)
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Old 03-06-2012, 06:38   #2
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Re: Yanmar 3HM35F thermostat question

I'm not sure this applies; but a stuck (open) thermostat could cause “white smoke”, due to low coolant temperature, resulting in incomplete fuel combustion.
This often occurs upon a cold start.

White smoke can occur as a result of combustion temperature being so low that only partial combustion occurs, and the partially burned fuel that exits the exhaust is in the form of a white vapoury smoke.
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Old 03-06-2012, 15:16   #3
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Re: Yanmar 3HM35F thermostat question

Yes, I understand that part of it. The stbd engine doesn't have a thermostat in it at all and it smokes white for a few minutes until it warms up. I would imagine a thermostat stuck open would cause the same effects.

What I'm seeing is an engine that starts and runs just fine for 1-2 hours and then starts running rough and emitting puffs of whitish/grayish smoke. It's not blue, and it's not black, so I guess it's white.

I was curious as to how you can tell an engine is overheated by observing operation, without any gauges to read temp.

I checked impeller yesterday, it's good. Like new. I did find the water pump belt was a bit on the loose side, and tightened that. I ran for amost four hours today at reduced RPM and it didn't miss a beat.
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Old 03-06-2012, 15:46   #4
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Re: Yanmar 3HM35F thermostat question

The easiest way to check is to remove the suspected thermostat and check if it is stuck in open, if it is in the closed position put it in warm water (80 deg C) and observe if its expanding. If it does not expand its faulty and needs to be replaced.
Push comes to shove you can run this engine without the thermostat until you get a new one.
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