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Confused about which oil weight to use

15K views 37 replies 9 participants last post by  Rusty123 
#1 ·
The shop manual for my Yanmar 2QM15 specifies either 10 or 20 straight weight oil, depending upon outside temps. I realize that when the manual was written, modern multi-weight oil didn't exist, but my understanding is that because the engine was designed to operate with 10 or 20 weight oil when at operating temperatures, I should use a multi-weight oil with a similar operating temperature weight (something like 5W20, for example).

Problem is that the readily available diesel oil doesn't seem to be available in this low viscosity. For example, Rotella T, which many recommend, comes in 15W40 and 10W30, both of which would seem to be too thick.

To add to my confusion, since this is a raw water cooled engine, it operates much cooler than a fresh water cooled engine, making the use of too thick of an oil even more of a problem.

What am I missing in this equation?
 
#31 ·
I can remember having salt water cooled Chrysler Crowns and Royals in some of my old boats. Those engines lasted decades with salt water cooling. Of course, the castings were twice the thickness of modern motors and made from non-recycled cast iron. After quite a few years, they would usually succumb to graphitization, crystallization of the iron, induced by salt water electrolysis. A lot of the new power boat V8 automotive conversions are very thin castings. I can't imagine them lasting more than a couple of years if cooled with salt water. The nice thing about the little Yanmar diesels is that they are more like the old straight 4/6/8 blocks of the past.
 
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#32 ·
WOW, such wonderful responses and insights from so many experienced sailors! I love it. I am learning so much about my motor! Simply Awesome.
This 3GMD YANMAR (raw water) 20 HP at 3400; 1981-83 built motor was installed on the boat (newly rebuilt) in 2004. I doubt if it has more than 300 hours on it. 150 hours have been put on it over the last 14 months. One 14 day trip from sarasota to Key West and Back. And recently, the trip up to Tampa, and back 20 hours over 5 days. So, As I mentioned earlier, I discovered that there was no water thermostat. I just bought one today, and I am hoping that the motor runs right when I put it on. The impeller is new, and the old one was good. It is my back up now. I am wondering if while sitting at the Venice dock, if some plastic bag didn;t get sucked toward the intake and prevent water coming in through the thruhaul. Because, after running the motor for 22 hours on new oil and filter, she has not gotten hot, or shut down, since that time. I am sending in a oil sample for analysis, just to see if there are any major issues that are derived.
Thanks to all for your wonderful help. I will keep you posted on how it goes.
(Oh, I checked the raw water strainer, and it flushed water out when I opened the thru haul seacock, so I am getting plenty of flow. It was clean).
:)
 
#33 ·
DixieJ, a very good idea to have your engine oil analysed. Hopefully it will give you peace of mind. The fact that the engine seized AFTER you shut it down may have saved it. They do heat soak a bit after shutdown - that is one reason to idle them for five minutes after a hard run.

The Yanmar GM series are very robust and survive well even in salt water. With the two that I have owned over the years I have probably exceeded 3000hours between them. Neither had hour meters and I tend not to worry about hours anyway - more about having a calendar-based maintenance schedule e.g. oil and filters every six months, zincs at 12 months, impeller at 18 months etc.

On both boats, for less than 50 bucks I fitted a 'T' piece and shut-off valve in the raw water inlet line to which I attach a garden hose for an end-of-season freshwater flush (the boat stays afloat in salt water all year and we don't have a freezing problem here). If you do this, it is absolutely ESSENTIAL that you start the engine on salt water and then only put fresh water in at a fairly low pressure, then shut off the salt water intake and regulate the fresh flow carefully so you don't over-pressurize. You will need to look at the exhaust outflow to gauge this. Run it in gear at 1500 to 2000 RPM for a good 15 to 20 minutes to give it a good flush, keeping a constant eye on the exhaust outflow. You want it to get up to operating temperature. Then with the engine at idle, after a few minutes to allow it to cool down, shut off the fresh water first then shut the engine down, NOT the other way round (unless you want water in the cylinders!). By taking the hose off at the dockside tap first, by the time I get back to the engine it has drawn most of the water in the hose through and I shut it down immediately. You don't want to run the impeller dry.
 
#34 ·
Good advice about flushing with fresh water. If it's going to sit for a while, it might be a good idea to flush with glycol or regular anti freeze to further protect the water jackets in the block. Whenever you lay the boat up for a period of time it's also a very good idea to stuff a rag in the exhaust so air can't get into the valves through the exhaust pipe and cause corrosion.
 
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#36 ·
Assuming your oil sample report is good, it would still be worth monitoring your engine's health for a while.
Buy or borrow one of those filter cutters (like a can opener). If you can't get one at an auto parts store, any place that sells tools to aircraft mechanics will have them. They are cheap enough.

At next oil change (which may be worth doing early - say after 50 hours), when you pull the filter, drain it for an hour, then cut it open. Lay the paper element out on a sheet of white paper under a bright light. Any metal fragments will show up. If some are present, send oil to same lab that did your last one for another report. If no metal visible, you are probably OK.

Either way, if the engine is still running OK, pull top off cold beer and chill out......
 
#37 ·
Sounds like a great idea. I will definitely do that! I put the thermostat in yesterday (which it probably never had one in it); and ran the motor for 10 min at 2300 rpm (no load), then cooled down at idle for 5 min. and she ran warm-to-touch; at both speeds. -the motor sounded like normal, and I had good water flow out of the exhaust. So far so good. It starts on first turn of the key, everytime. Thanks!!
 
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