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Old 29-05-2016, 05:19   #1
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Yanmar -- Thermostat in Oil Circuit?

After arriving in Finland with 3/4 tank of fuel, due to a thousand miles of wonderful sailing and almost no motoring, we are now burning it up motoring against the wind through the inshore fairways.

We noticed that the oil pressure suddenly went up from 3.8 to 4.3 -- while motoring steadily at 2100 RPM.

Why would it do that? Both figures are in the normal range, but it seems weird.

My 4JH3HTE has a lube oil-to-sea water heat exchanger -- which along with all the other heat exchangers I had professionally cleaned last year. Maybe there's a thermostat in the that circuit? Which opened when the oil got to a certain temperature, which cooled it back down again (depending on hysteresis of course of the thermostat), bumping up the pressure?

Or what else could have caused this? Seems weird.
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Old 29-05-2016, 07:58   #2
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Re: Yanmar -- Thermostat in Oil Circuit?

Dockhead,

I looked at the service manual for our 4JH4-HTE (that has an oil cooler like yours) and there is no thermostat in the oil circuit that I can see. If the pressure fluctuated between the higher and lower pressures over time, it could indicate your oil pressure sender is going bad. Constant higher pressure at operating temp could possibly indicate your oil filter backing up and going into bypass mode. Easy to check that option though, just change the filter and see if it makes a difference.

Worst case scenario is it could indicate a blocked oil passage somewhere in the engine which would be bad news as that can lead to damage pretty quickly if a part of the engine is starved of oil. Given your maintenance regimen, I would give this a low probability.

On our engine we only see the oil pressure drop as the engine warms up and then stabilize at just over 3 bar at operating RPMs. I don't ever recall the pressure going up...
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Old 29-05-2016, 18:55   #3
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Re: Yanmar -- Thermostat in Oil Circuit?

You could add a tee where the sending unit is and put in a manual gauge. There is a pressure relief valve in or near the oil pump that lets some oil escape when the oil is thick and cold. Since the valve is open at startup, a piece of dirt can get stuck between the valve and seat lowering pressure. Later oil flow washes the dirt out and the valve closes and raises pressure.
My boat has manual gauges in the engine room and electric ones on the bridge.
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Old 29-05-2016, 20:41   #4
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Re: Yanmar -- Thermostat in Oil Circuit?

When something is "weird" (rather than obviously bad), I first suspect the instrument.

If you search this forum, you'll find reports of oddly short lives for 4JH3 oil pressure senders.

I'd change the oil, filter, and sender. That will likely be the end of it.
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Old 29-05-2016, 21:30   #5
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Re: Yanmar -- Thermostat in Oil Circuit?

Thanks for all the good advice.

I'll be changing the oil and filter soon. I might change the sender as well.

It could also be that this (3.8 vs 4.3) is all just in the normal range of a not very accurate sender, it seems to me.
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Old 29-05-2016, 23:21   #6
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Re: Yanmar -- Thermostat in Oil Circuit?

Is the water colder there than you're used to? Cold water in the exchanger could be cooling the oil more than usual, colder oil could be causing higher pressures.


A competition engine I had once actually blew the line off the oil pressure gauge on a cold (below zero) morning. I was running a 50 grade oil, which gave nice oil pressure when it was hot, but was too thick in freezing temps.
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Old 30-05-2016, 01:46   #7
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Re: Yanmar -- Thermostat in Oil Circuit?

We have the same Yanmar engine and replaced three oil pressure sending units and the oil pressure gauge four years ago, only to find out after the changouts didn't fix the problem, that the issue was an electrical connection to the gauge.
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Old 30-05-2016, 09:10   #8
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Re: Yanmar -- Thermostat in Oil Circuit?

My response would be to change the filter see if the internal filter bypass is causing it.
Could be the pressure sender or loose connection failing the filter change.
Not sure re Yanmar but Universal/beta etc have a pressure relief valve in the engine block, may have an issue there but that'd be least likely.
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