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Old 31-03-2014, 11:28   #1
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Is a check valve before the lift pump common?

Hello all -

We've a Perkins 4-108 from the mid-'80s in our sailboat. We are looking to replumb the low pressure side of the fuel system as the compression fittings are failing.

One place we have a leak is at the inlet side of the lift pump, where the previous owners installed a check valve with no provision for a compression fitting. We are wondering if there is a need for a check valve at the lift pump inlet, and if so whether we can move it off the lift pump and onto the piping that feeds the lift pump.

For background, the fuel supply system is currently tank -> electric fuel pump -> racor -> check valve -> lift pump -> on-engine fuel filter. The tank is below the level of the engine lift pump.

Thanks!

Scott
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Old 31-03-2014, 17:31   #2
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Re: Is a check valve before the lift pump common?

I thought mechanical pumps usually functioned as a check valve by virtue of their design? IE there is an inlet and an exhaust valve in any diaphragm pump isn't there?
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Old 04-04-2014, 09:30   #3
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Re: Is a check valve before the lift pump common?

The mechanical fuel pump on a 107 is your standard kind of pump with a check valve in the inlet and outlet. One failure mode of mechanical pumps is the check valve. This sounds like someone has made an external repair on a faulty fuel pump.
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Old 04-04-2014, 09:42   #4
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Re: Is a check valve before the lift pump common?

Thanks. I did think that was the case.

We've replaced the lift pump, and will see how it works without the external check valve.

On to the next problem!
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Old 04-04-2014, 09:57   #5
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Re: Is a check valve before the lift pump common?

Agreed! You shouldn't need a check valve. If your inlet hose is in good shape and air tight on the fitting then it should hold fuel w/o leaking back into the tank.

Another thing you can do is run a loop in the fuel line above the lift pump between the elect. pump and Racor. Or even between the tank and elect. pump. This would require a tight fitting at the inlet of the elect. pump. If you have any other leaky fittings this will help.

I assume the elect. pump is inline for priming the filter and/or bleeding lines.
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Old 04-04-2014, 10:33   #6
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Re: Is a check valve before the lift pump common?

i'm with guy. someone tried to save a buck by not replacing the mechanical lift pump. really bad idea. a leak in that pump can put diesel in the crankcase.

my 4-108 system is exactly like yours except for the extra check valve. works great. i don't actually use the electric pump when running, only installed it to make bleeding easier. since i installed it two years ago i've never had to bleed the engine - go figure.
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Old 04-04-2014, 11:20   #7
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Re: Is a check valve before the lift pump common?

We've never had a problem with having to bleed the system during the season, but we do (as the previous owner suggested) leave the electric fuel pump on all the time.

Last off-season we did a lot of work to the low-pressure side of the fuel system, including replacing the lift pump (which was oozing). As a result of the work the compression fittings never did reseal properly. We tried to tighten down on one of the connections to the lift pump (an off-brand) too much and cracked the lift pump body and replaced the lift pump with a Perkins pump from TAD.

This off-season (as in right now) we are re-making the tubing so we can use new compression fittings; there are some questions there as well (as you can read about).

Thanks for all the help - the expertise on this site was invaluable last year when we were working on the injector pump governor.

Thanks!
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Old 04-04-2014, 11:40   #8
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Re: Is a check valve before the lift pump common?

Quote:
Originally Posted by sdynes View Post
...the fuel supply system is currently tank -> electric fuel pump -> racor -> check valve -> lift pump -> on-engine fuel filter. The tank is below the level of the engine lift pump.

Thanks!
Scott

Should go to the Racor first, then the lift pump. Why? You want to filter out before the pump. Many pumps, like the Facets have tiny screens on the bottom. You don't want that screen first in line.

http://c34.org/bbs/index.php/topic,5....html#msg32616
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Old 04-04-2014, 11:56   #9
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Re: Is a check valve before the lift pump common?

Hello Stu -

Do you mean the fuel should go to the Racor before the electric pump, or the lift pump? The lift pump is after the Racor, the (universal automotive) electric pump is before the Racor.

We use the electric pump to bleed the system up to the lift pump when we change filters.

Thanks!
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Old 04-04-2014, 11:59   #10
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Re: Is a check valve before the lift pump common?

Sounds like you didn't read the "link above", this one:

Complete Fuel Filtration - Another Little Story

Yes, tank, racor, pump...

You still use the pump to fill the system when bleeding. Doesn't matter where in the line the pump is. I put injector cleaner in the racor when I replace it, less messy than diesel.

Maybe I don't understand, you have two pumps? But all pumps should be after the racor for the reasons discussed in this link.
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Old 04-04-2014, 12:04   #11
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Re: Is a check valve before the lift pump common?

Oops- I did miss the link.

Thanks!
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Old 04-04-2014, 12:14   #12
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Re: Is a check valve before the lift pump common?

Do not use a check valve on a diesel fuel system. They are required for gasoline, but are a PITA in a diesel system.

I had one feeding a Yanmar 3gm30f. It worked for many years, but one day the engine started cutting out. I replaced the fuel filter but couldn't prime it. Turns out the check valve which was just before the tank was plugged with algae and junk. The tiny orifice of the check valve is a perfect place to trap crap. Removed the ball and spring, flushed it out and it has worked fine ever since.

David
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Old 04-04-2014, 19:33   #13
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Re: Is a check valve before the lift pump common?

As an FYI I put a Tee at the tank outlet > then to the elect. pump> then to a two way valve> then to the filter. By turning the valve I can direct fuel straight from the tank, or thru the elect. pump. Both go thru the filter.
Elect. pumps shouldn't need a filter unless the tank is dirty/old.


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