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post #11 of 14 Old 05-21-2015 Thread Starter
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Re: Fuel Pump Noise

Here's a good thread on fuel return to the tank. It sounds like it should be very little and I'm assuming that it only occurs while the engine is running.

https://www.sailnet.com/forums/diesel...fuel-tank.html

I guess it's time for me to plug the fuel line at the injector pump and see what happens. If the pump continues to run it should be either a faulty pump or air entrapment in the pump. Another thought is the possibility of it sucking air from either the pump strainer gasket or a fuel line upstream of the pump.
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post #12 of 14 Old 05-21-2015
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Re: Fuel Pump Noise

More than likely you have a 1987 Universal M25 in your Catalina. The picture above is of a Facet fuel lift pump. This pump has a filter in the bottom, by the way.

The fuel tank should have 1 petcock for the fuel OUT line. This line should go to the Facet fuel pump pictured. From there the fuel should go to the Raycor filter, and it does because I can see it in your pic. From the Raycor, the fuel should go to a second (finer) fuel filter, and then up to the high pressure injection pump.

Assuming that you have an 86, or newer M25, there is a bypass valve on the inlet to the high pressure injection pump. The bypass valve allows fuel to bypass the high pressure pump, and to directly enter the fuel injectors at relatively high volume, and low pressure. This valve is there to allow you to EASILY bleed the system. In normal operation, the valve should be closed, and the lift pump should "tick" at a rate of about 1hz. When the valve is opened for bleeding, the lift pump will "tick" at about 10hz.

When fuel gets to the high pressure pump, it is forced at very high pressure (1991-2300 PSI) and low volume into the metal injector lines at EXACTLY the right time as the crankshaft rotates. This pump is driven by a "fuel camshaft," and there is a procedure for fine tuning the timing if necessary. The high pressure forces the injector to open, and forces some fuel into the combustion chamber.

Some fuel, however, is supposed to leak past the injector nozzle, and is used to cool and clean the injector. This fuel, which misses the injector nozzle, backflows through the injector assembly, and enters into the fuel return line. The return line daisy chains across all three injectors, and then returns the unused fuel to the fuel tank. There is no petcock on this line.

Here is a GREAT video that explains the injector;


The M25 uses a pintle type injector.


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Last edited by eherlihy; 05-21-2015 at 02:38 PM.
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post #13 of 14 Old 05-21-2015 Thread Starter
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Re: Fuel Pump Noise

Thanks for all the info. I couldn't see the video, but I know the basic workings.

I knew about the lubing of the injector, but I didn't think it would be that much especially with the injectors not firing when the engine is off. I've got the spare pump now so it will probably last forever vs not having a spare.... I'm still going to test the pump with the line blocked off to see if it stops and I'll see if I can scrounge up a 0-10psi gauge to test the pressure.

I'll report my findings.
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Re: Fuel Pump Noise

You may have a bad bypass valve...


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