'79 Yanmar YSM-12 oil pump fails to self prime

[Kabloona]

Concerned persons:

Since it was "rebuilt" (piston, rings, valves, crank, bearings, seals, etc.) 2 years ago, the oil pump in my 1979 Yanmar YSM-12 (sailboat) diesel has lost its prime with increasing frequency. To get it pumping oil again each cold start, I have had to remove the triangular oil pump cover (revealing totally-dry pump cavities) and fill the pump cavities with oil. After quickly replacing the pump cover and 3 screws, and re-starting, oil pressure comes immediately up (to 70 psi cold, then it subsides to its running level of about 18 psi) and it'll run all day faultlessly thereafter. It has never failed to maintain pressure once it has it.
Guided by several experienced Yanmar mechanics, I have over time replaced both oil pump rotors and all related seals and gaskets, let the newly-started engine run with zero oil pressure (and the siren screaming) for up to one minute to see if it would pull up a prime on its own, and (finally last fall) taken the side of the engine to a diesel engine machine shop and had the pump shaft opening drilled out and bushed for a closer fit. But now, since the rotor shaft has gotten loser in its new bushing in just the 100-or-so hours of running since I had it re-bushed, I’m wondering whether the machine shop correctly positioned the new hole.

This spring (after a winter on the hard) it cold-started with immediate pressure and I thought my troubles were over. But it has since progressed from one failure to get any oil pressure (after sitting over night) per 10-15 starts at the beginning of the sailing season this year, to a failure once every few days, to current failures with every over-night sit. Again, it has been under 100 hours running time since having the pump shaft professionally bushed! As yet, it has lost its prime only once after a period of “sitting” as short as a day sail of only 4-6-hours.

I've currently got the side of the engine off again (for the third time) and am performing simulations with the oil pump-containing side sitting (at the correct angle) in a pan of oil at "running depth". With pump chambers even half full, it will usually regain a prime and pump oil up (past the filter and the pressure regulator) until it runs out the two feed holes: (1) the hole to the rest of the engine; and (2) the hole to the oil pressure meter.

How can I stop this drain-down?? Shouldn’t the pump be able to draw up its own prime even if it drains down between starts (it’s only a lift of about two inches in a ~¼” cylindrical “tube”)? Am I the only person ever to have this problem!!??

Big thanks to anyone who can give me any suggestions! Kabloona

 

[sand sailor]

losing its prime

probably a bad gasketon the pump leaking air into the system very close to it or in it or maybe the pick up tube is cracked and the pump is sucking air into it either way put z-max in the oil this stuff will save the bearings from failure until you get the problem solved

 

[Hermit Scott]

This is the only real problem with this little engine. After rebuilding mine. I noticed the oil pressure was 30 psi on start up and 20 psi if ran really hard. It's somewhere in between running it normally.
I have my doubts that anyone can drill and bush that pump housing with the accuracy required to make it work right. The worn out pump shaft bushing doesn't seem to be what's causing the failure to prime. From what I have read of others you are the first I have seen to have this problem.
Since you have the side off, and everything that could be wrong is in there, I would have it pressure tested though the oil passageways. Pressure test everything. You have to have a crack somewhere.
My ysm is still chuggin' along for now. I bought an old 2QM20 to rebuild after I found out about the oil pump housing problem on the ysm12.
I hope you find the demon in that engine.
Have you done any pressure tests in the passageways?

 

[Bill Roosa]

What brand of oil are you using?

not weight but brand.
I used pensoil in my old VW and the thing developed leaks in all the seams. Kinda scary to watch the oil just start leaking from seams that where dry before. Put quakerstate back in and the leaks stopped.
Not that I'm trying to rag on one brand or another but some don't work in certain applications. Pensoil high detergent oil is not recommended for dirty old VWs as it cleans the gunk out of the cracks and makes it leak.
Perhaps something similar is going on here?

FWIW

 

[Kabloona]

Hermit, You are an amazing asset. I have read and appreciated several of your replies to others' questions. I really appreciate you coming back so quickly with a well-thought-out response.

I see in my Service Manual for the YSM engines that, although the illustrations don't show it, the wording refers to the oil pump for the YSM-12 as made up of not only an inner rotor and an outer rotor but also a replaceable "pump body"!!! While I have replaced both rotors and all seals, I only just now noticed that the wording suggests replacing the two rotors and the body as a "set". If this set is available, my troubles may be over. I can't call my supplier until Tuesday. I'm very excited at this possibility. Do you know if the pump body is removeable/replaceable too and comes as a set (i.e., no need to have drilled and bushed the shaft opening as I did last year - to no avail)??

Thanks for your concern. Kabloona

 

[Hermit Scott]

The reason why you had to bush the rotor shaft is because the side of the case is not available. The manual says to replace the outer rotor and pump body as a set. The bad news is the "pump body" looks to be the side case it's self.
If that is your problem, there is one other option for you; install an external oil pump, belt driven or maybe electric(would be easier). Some one else suggested that somewhere else on the internet, I can't recall where.
Drill and tap the pumpshaft hole, put a pipe in there. Drill and tap into the oil line that feeds up to the blowoff ball and spring and plug off the entrance to the pump body and put a pipe in there.
This seems extreme, but if the problem is the side case and there are none available, It's either try this or replace the engine.

 

[Kabloona]

Hermit,
Two more questions. As you understand the YSM-12's "low-pressure variation trochoid oil pump" (rotary pump) how is it designed to gain a prime after each period of sitting idle. Is it designed to maintain its fully-oil-glutted state until its started the next time, or is it designed to have sufficient air pumping ability to create a vaccuum and pull up oil from the pan to re-prime inself? In the latter case, in the YSM-12, it would only have to suck up oil about 2" in a cylendrical passage that's less than 1/4" in diameter.

Finally, do you know of an authoritative tech rep. web source that could supply me with the design specs for the I.D / O. D. variance allowed for installing a new bushing for the inner rotor shaft through the back of the oil pump cup? If it turns out ( as you say) that the pump "body" is not removeable/replaceable from the cast side of the engine, I am ging to try to do it AGAIN! But I'm a little leary of doing this since the professional machine shop's try less thasn 100 hours ago has failed already.

Thanks again for your concern, Kabloona

 

[Hermit Scott]

It actually has to suck the oil up much less than 2". It only matters where the oil level is not that the dip tube goes to the bottom of the oil pan. For example if you stuck a straw in the ocean 40' long and tried to suck up water at the surface it would be as easy as if you used a 6" straw.
You need to have someone measure ALL the tolerances on the pump and housing to know for sure what the problem is. If it's out of spec, where and by how much?

I believe the pump is designed to prime it's self, but I am not an expert on this type pump. I just learned how these pumps worked when I rebuilt this engine.

If you have new pump parts and the hole for the bushing was drilled the correct distance from the pump body wall, then the tolerances should be close. The problem is you can't do anything about the pump body wear. I guess you could bush that, but then you have two questionable holes adding up the misalignments.
Unless you could bush the pump body measuring off the pump shaft hole. The problem still remains that you don't know if the pump shaft, in it's new hole, is spinning concentrically with the shaft driving it. If it's not it will cause premature failure of the new bushing.
If I ever have this problem on mine, I won't try to fix the old pump. I will look for a replacement electric and drill and tap lines into that side cover for external lines.
If it was a belt driven it would variably pump more as the engine increases in RPM, but I don't think there would be any harm in pumping too much oil pressure for low RPMs, but I am not sure. I would have to ask my dad. He is the head millwright at a paper company. He knows everything.

And lastly I read about some one who heard from a former friend of a friend that there was a machine shop iin the UK that has a jig to drill and bush the YSM12 cover. If you get bored you could go on a quest to find it.

 

[Hermit Scott]

Looking at the diagram in the book, it appears that the cam shaft that drives it has a little cup that it fits in the cover. If I were going to try drilling out the pumpshaft hole to rebush it, I would make a jig on a lathe that just fit into that cam shaft cup with a hole in the center of that the size of OD of the press fit bushiing. I would use a mill not a drill bit so it won't walk into the old hole.
I am not a machinist by any means. Again, I would consult someone who I trust who knows about machining to see if this is possible with any accuracy.

 

[Jalepeno]

YSM-12 oil pressure

Kabloona,
I have a YSM-12 in my CAL2-25 and find it a very simple and reliable motor. I don't know the number of hours that were on it when I bought the boat.
This may sound like silly questions but have you checked the tolerances between the pump inner and outer rotors, between the outer roter and the housing (cylinder side cover) and between the rotor and the cover plate? If so, how do they compare to the limits shown in the manual?
Also what type and weight of oil are you using?
Also have you checked the oil pressure regulator spring?
I can post a copy of the YSM-8/12 manual if you don't have one.
s/v Jalepeno

 

[Kabloona]

Jalepeno, Thanks for your reply to my questions re: my YSM-12. Yes, I have the shop manual and have measured the three offsets it suggests. Since both rotors are new (last year), their dims are good. Even the offset to the original body side is within spec. I'm using 30 wt. oil for diesel engines. Since the oil pressure isn't my imediate concern, I haven't replaced the presure spring, but it and the ball bearing in the pressure regulator valve look in tact.

 

[Douglast]

im not familiar at all with the yanmar...but i must ask one question...is there not a oneway check valve or foot valve that prevents the oil from draining back from the pump to the oil pan to keep the pump cavity and intake line charged at all times..??..or at least the pump intake cavity..??

 

[Hermit Scott]

There is no check valve. The pump sits nearly at the level of the oil.
I have to think that when you start to really look at this, there is going to ba a glaring out of tolerance or crack somewhere hidden in that side case.

 

[Jalepeno]

Kabloona,
If all of your pump housing, gears and cover plate clearances are within specifications (see Chapter #6: 0.002 to 0.004") then your problem may not be with the pump but with either an air leak in the oil suction tube connection, the pump shaft or your oil level / type of oil. As far as I can see in the manual, the pump shaft should be self aligning (within reason) with the cam shaft and there is no check valve in the suction tube. The oil pressure regulator (ball and spring) should maintain the oil pressure between 35 and 50 psi above idle speed. The low pressure alarm switch (idiot light and alarm buzzer) turns on when the pressure is between 1.5 and 4.3 psi. This is very low and you should not run the motor at such a low pressure. My oil alarm turns off almost as soon as I engage the start motor.
My Yanmar service centre recommends 3 litres of Volvo-Penta VDS-3, 15W40 API CI-4 oil (see Chapter #13). A 30W oil may not be the best choice for our climate. By the way he said never to use synthetic oil in an older engine.
s/v Jalepeno

 

[Hermit Scott]

I use synthetic in my YSM12 but only after I rebuilt it. I have no leaks and it doesn't burn oil. I use rotella 15-40 synthetic.
I put a synthetic blend in my car at 180,000 miles. It was 2 quarts low in a very short time. I switched back to regular oil and I don't have any problems with it buring oil.
Jalapeno-do you have an oil pressure gauge on your engine, or just the low pressure alarm?

 

[kloudie1]

It has to be an air leak somewhere above the pump in order for air to get into the pump itself.. I’d suspect the easy ones first.. The O-ring on the cover needs to be very clean, as does its groove and the surface it touches on the cover. The top banjo fittings, the gasket face and gasket between the gear case and cover, the oil pressure switch, the oil pressure controller and its ball valve.... The filter cover o-ring and the cleaning shaft o-ring that seals the shaft where it comes out of the filter cover..even though it is below the level of the pump. Like Hermit says, a good machine shop/machinist can get that drive hole for the pump exactly right with a milling machine. He would figure the exact center of the big hole in the case cover then determine the tip clearance of the inner rotor to the outer rotor, the thickness of the outer rotor at the tip pocket, the radius of the inner rotor out to the tip. Then he’d offset the drive hole by the dimension that gives the correct tip clearance.. Not too hard with a digital indicating mill. They could bush both the big hole in the case, and/or the little hole to restore the dimensions of the pump, if need be. It would probably cost a coupla hundred or a bit more to do that.. and they’d have to agree to hold the dimensions to tolerance.. no problem for a good shop and machinist.

 

[Jalepeno]

Scott,
I only have the original oil pressure switch at this time. I will be adding a simple 0-100 psi hydraulic mechanical pressure guage on a T-fitting with the switch as soon as I can figure out the thread sizes. It looks like metric pipe threads are the same or almost the same as NPT threads. Will a NPT fitting work?
s/v Jalepeno

 

[Hermit Scott]

1/8" is the thread hole right on top of the aluminum side case. I know it doesn't look like 1/8" but that is what it is.
I don't have an alarm right now. All I have is a pressure gauge mounted facing up right there at that top hole.
The gauges are 1/4" thread as far as I have seen. I bought a 90 degree fitting that was male 1/8" with a female 1/4" threw some tape on it and I was in business.

 

[Jalepeno]

Scott,
Thanks for the thread info.
s/v Jalepeno