Yanmar YSB Oil Pressure problems - The full story!?
Abstract: I've seen quite some remarks about these old Yanmar on
the net. And after investigation of (inside) my engine i've made som
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<>In the fall of 2000 I got a suprise: The oil pressure warninglight
was flashing weakly while engine was running on idle speed. But only
when engine was hot. So after changing the sensor for the lamp,
cleaning the oil-filter and even changing the oil it was still flashing.
I went thrue all normal checkpoints:
Blue smoke from engine: No,
only black smoke at full speed - normal (making noise and vibes as a
Oil consumtion high: No seems
not to burn any oil at all.
Oil in the engine room: Nope.
Oilfilter failure: No -
Worn out oil pump: No -
clearence for the moving parts is ok. The lid had only some small marks.
Over heating: No, not even
after several hours of "high" speed, 6 knots.
So after some chats with some Yanmar experts it seems to be the oil
pump it self that needed to be replaced - They tought! Wrong boys!
These Yanmar YSB engines has an oil-pump that dosn't has a separate
bearing for the drive shaft. It's rotating direct into the engines
aluminum frontcover and that gets worn after many years. A removal of
the cover and some days at "BRISHÄLLS Motor" in Göteborg gave
engine an oil pump witch was "better then new".
Thank you Brishäll! Good work.
How it looked Before
Click for a large close up
To the left is the old rotating oil-pump parts. Only small marks
at A and B, was noticed on these parts.
No real meaning to change the pump parts, but I did it anyway. On the
drawing to the right you will see the "problem". The oil, C, is leaking back into the
engine at the
shaft hole D. So to fix it
Brishäll drilled up this hole and mounted a small brass bearing,
yellow at D.
Bearing in place - from the outside.
And from the inside.
For my engine, it did not help with a new oilpump, new lid and a new
shaft bearing. Still low oil pressure at idle RPM.
During summer 2001 we run the engine quite a lot, ( Due to
no-wind-and-very-hot-weather :-) back and forth to
Denmark > 150Nm at engine. Normal speed/Rpm 4-5 knots. No problem
with heating etc. No oil consumtion.
So the truth is probably inside the engine. A partly broken main
bearing perhaps. I will take the engine apart this winter 2001/2002.
Yes, took it apart completely. The rear crankshaft bearing was worn
down to the copper witch is a bit to mutch... After dismantling the
gearbox I could move the balance wheel up and down 2 mm!
The YSB 12 engine is an extreamly robust one. It's heavy, 140Kg, it
have thick iron walls everywhere. It will last for 100 years - with
The crankshaft was check by ??-motor. The hardening was ok but some
scratches was takan away and the new bearing was fit to the crackshaft
- Splendid work!!! The front crankshaft- and pistonbearings was
replaced. And also the other ballbearings at the front of the engine.
None of this bearings was really worn but now was the proper time to
Why have this happned? The previous owner had let some amateur to mount
a freshwater cooling system witch he had done wrong in some way and the
engine was running to hot and stopped. The previous owner had then
replaced the cylinder and piston as well as a the whole top! So I
examine the valves and piston - all was like new - except for one
pistonring that was broken. Tiny scratch marks in cylinder only no
The exthaust iron pipe had cracked and was leaking gas and dust as you
can see on the first picture. While engine was out I cleaned and
painted the engine room. Glued noise dampening stuff on the whole
engine room and hatch. New bearing, and gaskets for propeller shaft.
New exhaust rubber tube. All electrical wires was also renewed.
Improved the electrical grounding between Generator/Starter/batteries.
I also mounted the freshwater cooling stuff I got with the boat (!?).
There is probably no need for freshwater cooling on these kind of
engines. The are so robust that they probably only need a new top every
25 year so.
Anyway, a freshwater heat-exchanger should be placed as low as
possible. If placed in the "same hight" as the engine and low
internal water level, the circulation pump will not work due to air in
the water. The engine is overheated quite quick. The expansion "bucket"
is then placed as high as possible for easy ventilation of air from the
cooling water. DON'T CHANGE THERMOSTAT TO A HIGHER TEMPERATURE. Keep
the old one - about 50 degree Celsius. If you mount a thermostat with,
say 90 degree then you probably need a Oil-Cooler. The only thing that
cools the oil is the surrounding air and the cooling water inside the
engine. So after several hours of hard engine run in a very hot day,
the oil is getting very hot and thin. You don't see the oil temperature on the
watertemp meter! My engineroom get really warm but the heat stays
inside the engine room thanks to the isolation. I measured the oil
temperature after a long engine run ( from Kristiansand, Norway,
to Skagen in Denmark approx. 100 Nm ) and it was around 90 degrees C.
witch is quite perfect. I did some thinking/priceing/work checking on
installing a oil cooler but that's a lot of work.
The last and the most important thing is the oil pump. The Japanees
may have created a everlasting engine, but why did they place the oil
the normal oil level!!! The problem arise when you have Hot engine and
a worn pump house and or pumplid. the oilpump is "sucking air"! You
wont get any oilpresseure unless you run with high rpms och wait for
the oil to cool and therebye improving the pump function. Se sketch
below. I the pump had been below oillevel all the time then the oil
pump would not get worn until the hole engine is worn out.
The red arrow to the right symbols the path of the oil, from high
pressure side back to lowpressure without doing any good. Likewise,
during start moment with hot engine there is air flowing the in the
same way and preventing the pump to rise oil from below.
To fix this - witch is quite easy if not too worn : Check the "Purple"
distance above and file down the front engine lid so the "purple"
distance dissapeare but pump still can rotate easy. Also check the
bottom och pump house, purple circles to the left. Warning: To file
down something like this you need to have some skills...You could also
ruin your engine. For some engines a complete new front lid may be the
So, my engine will last 25 trustful years more. This repair cost me in
around $1800, then I got a nice engine room too. A new two cylinder
Yanmar with more power and less noice and vibrations and same weight,
would be at least $7000, and thats is about 1/4 of the boats whole
End Of Story.
I will try to add some pictures here on my "new" engine as soon as
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