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Old 14-03-2020, 18:03   #1
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Yanmar YSM8 Leaking Fuel Injection Pump

We've been diagnosing the cause of a recent runaway on our Yanmar YSM8. The engine was shut down with the decompression level, and we think (hope) it was before any major damage was done.
  • The oil had been at normal level before runaway
  • After runaway, the oil level was overfilled to the point of spilling out the dipstick hole
  • The oil in the crankcase was thinned out and had tons of diesel in it
  • Drained the oil, reset to lowest dipstick reading, and turned over the engine
  • Fuel is leaking into the crankcase
  • No fuel is making it to the injector (disconnected, turned over engine, no fuel leaves high-pressure hose)
  • Confirmed the fuel lift pump is sending fuel, so suspect this isn't the issue

From the troubleshooting we've done, we suspect the integrated fuel injector pump is malfunctioning, and leaking diesel into the crank case rather than sending fuel to the injector.

For anyone more familiar with these older Yanmars, does that sound right? If so, is it worth trying to rebuild the injector pump, or having it replaced? They no longer seem to make the part...
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Old 14-03-2020, 18:42   #2
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Re: Yanmar YSM8 Leaking Fuel Injection Pump

Quote:
Originally Posted by zackliscio View Post
We've been diagnosing the cause of a recent runaway on our Yanmar YSM8. The engine was shut down with the decompression level, and we think (hope) it was before any major damage was done.
  • The oil had been at normal level before runaway
  • After runaway, the oil level was overfilled to the point of spilling out the dipstick hole
  • The oil in the crankcase was thinned out and had tons of diesel in it
  • Drained the oil, reset to lowest dipstick reading, and turned over the engine
  • Fuel is leaking into the crankcase
  • No fuel is making it to the injector (disconnected, turned over engine, no fuel leaves high-pressure hose)
  • Confirmed the fuel lift pump is sending fuel, so suspect this isn't the issue

From the troubleshooting we've done, we suspect the integrated fuel injector pump is malfunctioning, and leaking diesel into the crank case rather than sending fuel to the injector.

For anyone more familiar with these older Yanmars, does that sound right? If so, is it worth trying to rebuild the injector pump, or having it replaced? They no longer seem to make the part...
Welcome aboard CF, zackliscio. You will find enough information on these older Yanmar's (Y and S series etc) on CF to keep you reading for a week!

Leaving aside the run away issue, the injector pump can be rebuilt. There are only two (maybe three) parts inside that will wear significantly - the plunger (and plunger barrel) and the delivery valve. These parts should be available. They are available for the earlier YSE8 so I think they will be available for the YSM8. Note, the YSM and YSE use different injector pumps and as a generalisation, the YSM pump is better. Both can be rebuilt by anyone with some mechanical hand skills, a modicum of intelligence and the service manual.

The member you want on CF is Compass790, he has quite some experience on the pump. Currently he is away (probably fishing ). I suspect he will be back within a week or so.

As for the fuel contamination in the oil, it is possible the fuel lift pump is both pumping fuel to the injector pump and leaking into the crankcase. Pull it off, inspect the diaphragm and manually operate it on the bench to ensure the diaphragm isn't leaking. Close off the outlet side with a bung (or finger) to make it a little pressurised when testing it.

Let me know if you need a pdf of the service manual.
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Old 14-03-2020, 18:46   #3
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Re: Yanmar YSM8 Leaking Fuel Injection Pump

Thanks so much for the warm welcome and great response. Long time lurker on these forums, but first time poster

Great call on checking the diaphragm (hadn't considered it could be failing and sending fuel). We'll pull that next and see if we can find more info there. I'll some pics too.
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Old 14-03-2020, 19:38   #4
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Re: Yanmar YSM8 Leaking Fuel Injection Pump

Good on you for having the presence of mind to get to the decompression lever! A diesel engine on full runaway can be a truly terrifying beast.

The fuel lift pump diaphragm is a prime culprit for large amounts of fuel in the oil.

For future information, diesel injection pumps can most certainly be rebuilt. It is not something that anybody can do well, it really takes specialized tools and test benches that few shops have.

This is really a job for a specialist. You'll know you have a good injection shop when you walk in and it looks as clean as an operating room, and the guys working there do NOT have grease under their fingernails!
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Old 15-03-2020, 09:44   #5
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Re: Yanmar YSM8 Leaking Fuel Injection Pump

Where are you located Zack? My genius mechanic in Bradenton, FL just fixed a similar issue for me, quickly and easily.
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Old 15-03-2020, 10:08   #6
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Re: Yanmar YSM8 Leaking Fuel Injection Pump

We're in San Francisco. There are a few Yanmar mechanics around, so I think we'll likely bring the injection pump to one of them if they're willing to take a look. Will definitely test the lift pump too at this point. I'll circle back with more info to help anyone else who has similar issues in the future.
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Old 15-03-2020, 19:16   #7
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Re: Yanmar YSM8 Leaking Fuel Injection Pump

Zack, here are some comments (after giving your reported symptoms some more thought).

Prior to the run away, your injector pump (IP) was working well enough to be operational i.e. pumping a metered amount of fuel at injector pressure (i.e. >2,000psi).

The pump is a simple (albeit a precision) arrangement of a steel plunger inside a steel barrel. While it might have been leaking diesel into the crankcase, it would be most unusual for it to fail to deliver any fuel when the injector has been disconnected. If the IP had been working at 2,000 psi, it would still be working at atmospheric pressure. I suspect your testing of it wasn't up to scratch. IMO, try again making sure it has fuel on the inlet side, the throttle is set wide open (i.e. WOT), the engine is decompressed, the injector is disconnected and the engine turned over on the starter motor. I pretty sure you will get a small amount of fuel pushed though every second revolution. If you close the throttle, you won't see any fuel (or only an extremely small amount)

I concur with billknny that the lift pump is the prime suspect for large amounts of fuel in the oil. The lift pump can deliver way more fuel than the IP needs so it very possible that even if lift pump was pushing 90% of the fuel into the crankcase, the remaining 10% is still enough for the IP to operated normally.

I disagree that the YSM IP is really a job for specialist. Of course a specialist is the best option and if your pockets are deep enough, then that is the way to go.. Also other IP's are more complicated than the YSM IP and may not be a DIY item.



But...

You can do a rough and ready test yourself though. Simply use the injector as the test bench. With the injector out of the engine but still connected to the IP, turn the engine over and observe the spray pattern. If it matches what the manual says then the IP (and injector) is good enough to run the engine - maybe not perfect but good enough. If you replace the plunger and barrel and the delivery valve with new parts, you can reasonably expect the IP to operate to as new specs.
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Old 16-03-2020, 13:47   #8
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Re: Yanmar YSM8 Leaking Fuel Injection Pump

Ah, when we tested it we didn't have the engine decompressed, and only had the throttle at about 75%. We've ordered a new fuel lift pump, suspecting that's a likely culprit, and will re-run the injection pump. For now we're on mandatory shelter-in-place for SF, so it may be a few weeks until I have an update. Thanks again for the awesome info!
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Old 16-03-2020, 16:22   #9
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Re: Yanmar YSM8 Leaking Fuel Injection Pump

^^ looking forward to hearing about it when you get it fixed and hopefully this disruption will be over in the coming weeks / months otherwise... it isn't going to be good for any of us.
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Old 17-03-2020, 07:57   #10
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Re: Yanmar YSM8 Leaking Fuel Injection Pump

It is normal for there to be a slight leakage from this type of pump into the crankcase as this leakage is the means of lubricating the plunger. This is why annual oil changes are required for this type of engine. How long since your previous oil change?

I would suggest you remove the fuel pump and take it to a diesel injection specialist so the leakage rate be checked If there has been a major leakage to the engine sump then as others have said it is much more likely to have come from the lift pump.
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Old 19-03-2020, 21:55   #11
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Re: Yanmar YSM8 Leaking Fuel Injection Pump

The leakage into the crankcase was major, but hadn't been pre-runaway. Replaced the lift pump and it seems to have fixed the leak, but the engine still doesn't want to start. Bled the lines after replacing the lift pump. Re-tested the fuel injection pump (thanks for the great info here) and it is delivering fuel, though a small trickle at most. Our next thought was to pull the injector and see if it's spraying properly. If it is delivering fuel properly, anything other obvious culprits before suspecting the issue is in the cylinder?
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Old 19-03-2020, 22:02   #12
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Re: Yanmar YSM8 Leaking Fuel Injection Pump

Have you bleed the high pressure pipe from the injector pump to the injector at the injector end?

If not, do so.

If still no start, pull the injector to ensure it's is spraying.

If so, then you either have no air or poor compression.

While you are at it, do check there is no constriction in the exhaust at the mixing elbow.

Good luck!
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Old 19-03-2020, 22:04   #13
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Re: Yanmar YSM8 Leaking Fuel Injection Pump

Presumably you haven't removed the injector pump but if you have, make sure the timing shims were put back correctly otherwise the timing could be off.
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Old 19-03-2020, 22:11   #14
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Re: Yanmar YSM8 Leaking Fuel Injection Pump

FWIW, there is a bit of a knack to bleeding the high pressure pipe at the injector.

Professional diesel mechanics may have better techniques but as DIY guy, if it necessary to bleed the pipe, I decompress the engine, spin it over on the starter motor and slightly crack the nut on the injector and then tightened again repeatedly until engine fires. Sometimes I have to crack it further than other times

I'm sure there is proper way though!
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Old 20-03-2020, 06:28   #15
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Re: Yanmar YSM8 Leaking Fuel Injection Pump

Quote:
Originally Posted by zackliscio View Post
The leakage into the crankcase was major, but hadn't been pre-runaway. Replaced the lift pump and it seems to have fixed the leak, but the engine still doesn't want to start. Bled the lines after replacing the lift pump. Re-tested the fuel injection pump (thanks for the great info here) and it is delivering fuel, though a small trickle at most. Our next thought was to pull the injector and see if it's spraying properly. If it is delivering fuel properly, anything other obvious culprits before suspecting the issue is in the cylinder?
The actual amount of fuel pumped each stroke is small and only develops the high pressure when the system is completely primed and delivering to the injector. One thing that might have happened when removing the pump and replaced it is the possibility that you failed to engage the forked rod inside the engine which engages with the ball on the pump rack (item 13 in drawing in earlier post.

To prime you connect the pipe from the pump to injector and leave the connection to injector slightly loose. once only fuel comes from this point when the engine is turned either manually or with the starter tighten the injector pipe nut With 1/2 throttle set when now winding the engine you should hear a squeak from the injector. This is easier to hear if cranking by hand. If no squeak then the injector could need servicing and the engine might be smoky when run.
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