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Old 23-07-2020, 16:47   #1
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Quick Diesel engine question

Yanmar 3HM35F engine. It has two fuel pumps, one on the engine and an electric one in line.

If the electric one fails during normal engine operation, should the engine side fuel lift pump be able to pull the fuel through the failed pump? If it doesn't, does it mean it's faulty too? Or could there be any other reason why it doesn't?

Thank you.
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Old 23-07-2020, 16:58   #2
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Re: Quick Diesel engine question

The answer to your question is generally yes the mechanical pump would be able to pull through with electric pump. I wonder why your motor needs to fuel pumps? Is the electric pump a factory-installed unit?
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Old 23-07-2020, 17:17   #3
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Re: Quick Diesel engine question

I would personally find it hard to believe that the engine lift pump would be able to draw through a non rotating electric pump. If the electric pump has been in use and failed, you would best bypass it since you may otherwise fatigue the diaphragm in the engine lift pump trying to suck through it.


Many smaller diesels use solenoid type piston pumps to lift fuel to the injector pump, in that case there is no engine side fuel pump, just the electric lift pump. They operate at a fixed frequency and I believe have a relief valve to keep supply pressure to the injector pump constant.
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Old 23-07-2020, 17:22   #4
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Re: Quick Diesel engine question

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Originally Posted by ahun View Post
Yanmar 3HM35F engine. It has two fuel pumps, one on the engine and an electric one in line.

If the electric one fails during normal engine operation, should the engine side fuel lift pump be able to pull the fuel through the failed pump? If it doesn't, does it mean it's faulty too? Or could there be any other reason why it doesn't?

Thank you.
It depends on the lift required. The engine lift pump only has a certain range of lift and if that is exceeded, it requires another pump closer to the tank.

Unfortunately I can't recall the specs for Yanmar small engine lift pumps but I sort of remember it is around 1 metre or 3 feet. I'm sure if this is wrong, someone will chime it with the correct figure!
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Old 23-07-2020, 17:31   #5
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Re: Quick Diesel engine question

the electric was most likely installed for bleeding. Some of them (like specific facet solid state pumps) are specified as 'pass-through'. they sit in-line just fine. Power them up, and you have Insta-Bleed.

Beats the hell out of flipping the little lever on the lift pump a million times.....

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Old 23-07-2020, 17:52   #6
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Re: Quick Diesel engine question

The engine is somewhat higher than the fuel tank, but it's inches more like feet. I like the electric pump for bleeding and redundancy, but if it stops the flow in case of failing, then the later function doesn't work.
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Old 23-07-2020, 18:04   #7
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Re: Quick Diesel engine question

Easy test - with engine off, crack open your bleeder on the filter and pump the little lever on the lift pump.....fuel out, electric is pass-through.

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Old 23-07-2020, 18:10   #8
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Re: Quick Diesel engine question

Its critical that the electric pump have the right characteristics. A low pressure higher flow type device. It is possible to buy a great many electric fuel pumps at the auto parts store or online, but if the delivery pressure is too high it is entirely possible to eventually fail the engine lift pump and fill the crankcase with fuel.


Best you try to determine exactly what electric pump you have and determine if it has the features that are needed (like flow through if disabled) as well as a delivery pressure compatible with your main fuel pump. Replace if needed and carry a spare.
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Old 23-07-2020, 19:37   #9
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Re: Quick Diesel engine question

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I would personally find it hard to believe that the engine lift pump would be able to draw through a non rotating electric pump. If the electric pump has been in use and failed, you would best bypass it since you may otherwise fatigue the diaphragm in the engine lift pump trying to suck through it.


Many smaller diesels use solenoid type piston pumps to lift fuel to the injector pump, in that case there is no engine side fuel pump, just the electric lift pump. They operate at a fixed frequency and I believe have a relief valve to keep supply pressure to the injector pump constant.
They can easily suck through a non running electric pump, all of us that have electric pump plumbed in for bleeding but donít leave them on when the engine is running have our lift pumps sucking fuel through a non running electric pump.
Many, many boats are set up this way, mine was from the factory.
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Old 24-07-2020, 12:12   #10
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Re: Quick Diesel engine question

I always put an electric in line for bleeding and filter changes on all my engines for about 60 years. I've never had a problem with pulling or pushing thru the other pump. And the electric can function as a lift pump if the engine mounted pump fails. Many people delete the engine mounted pump because if the diaphragm fails, it can pump fuel into your crankcase.
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Old 24-07-2020, 12:15   #11
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Re: Quick Diesel engine question

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Originally Posted by Westcliffe01 View Post
I would personally find it hard to believe that the engine lift pump would be able to draw through a non rotating electric pump. If the electric pump has been in use and failed, you would best bypass it since you may otherwise fatigue the diaphragm in the engine lift pump trying to suck through it.


Many smaller diesels use solenoid type piston pumps to lift fuel to the injector pump, in that case there is no engine side fuel pump, just the electric lift pump. They operate at a fixed frequency and I believe have a relief valve to keep supply pressure to the injector pump constant.
They do. Draw right through a pulse pump. Done it. Many guys on this forum have that type of pump in line for filter servicing.
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Old 24-07-2020, 13:42   #12
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Re: Quick Diesel engine question

Do you have any evidence that the pump being referenced is the "right" kind of pump ? Because 99.9% of pumps that one can buy at the auto parts store do not work like this. Most of them are simply twin gear pumps and you will not suck fuel through one of them if they are not rotating...


If the OP identifies the pump as being of the correct type, then great.



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They do. Draw right through a pulse pump. Done it. Many guys on this forum have that type of pump in line for filter servicing.
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Old 24-07-2020, 13:45   #13
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Re: Quick Diesel engine question

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Do you have any evidence that the pump being referenced is the "right" kind of pump ? Because 99.9% of pumps that one can buy at the auto parts store do not work like this. Most of them are simply twin gear pumps and you will not suck fuel through one of them if they are not rotating...


If the OP identifies the pump as being of the correct type, then great.
All mine in fact, were bought at NAPA auto parts for like $20. Had them on 4 boats. Yanmar, Volvo, Perkins and Mercedes engines.
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Old 24-07-2020, 15:29   #14
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Re: Quick Diesel engine question

Well, I can't test it unfortunately, as I determined that the engine mounted fuel lift pump failed, needs to be replaced. Not sure if this is the reason of the electric fuel pump, or the PO put the electric pump because it was already faulty. Will do the testing when I get my new sets of pumps.
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Old 24-07-2020, 17:24   #15
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Re: Quick Diesel engine question

Our electric pass through was added by a PO instead of replacing the mechanical (still on the list). I have seen them suggested as a convenience to bleeding the system. Ours has a switch at the engine compartment for that purpose.

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