Help me understand my fuel system

[hd002e]

Hello folks,

On my sailboat there is a westerbeke universal m35 engine. A while back the engine couldn’t start and I’m trying to figure out how the fuel system works.

The primary fuel tank supplies diesel to the day tank that is located pretty high up in the boat. From the day tank the fuel goes through a primary RACOR filter. And then to a WESTERBEKE 39275 fuel lift pump. After that pump into a secondary filter and then off to the high pressure pump and injectors. I think this is pretty standard.

The part that has confused me and the mechanic here is that the WESTERBEKE fuel lift pump does not turn on with the ignition but it turns on with the glow plug switch. So unless the operator engages the glow plug for a cold start the fuel lift pump does not do anything.

Would the gravity feed from the day tank (which is about 5-6 feet higher than the secondary filter) provide enough pressure for the motor to run without an ignition switched pump feeding into the secondary filter?

The mechanic is convinced that the WESTERBEKE lift pump needs to be wired to run at all times while the ognition is on but that’s not how the boat has been running for decades.

Thank you for any help you guys can provide!

[bad.wabbit]

Physical evidence indicates you don't need the lift pump to run the engine. The way it has been wired it looks more like the pump would be used occasionally, possibly to clear out air out of the fuel line. Which makes me wonder if the glow plugs actually work and switch was repurposed to just run the pump.

[Tsuru]

Our last boat had a Universal M35B that used the same lift pump. It was wired to run all the time, but the leads ran through the oil pressure sender. If oil pressure was lost, power to the lift pump was cut. This was done to protect the engine from loss of oil pressure damage. Since the engine has no oil pressure prior to startup there was a bypass, which was activated when the glow plugs were engaged. To start you moved the key to glow plug position and the lift pump engaged. When the engine starts the oil pressure is restored and the lift pump continues to run.

At one point we had a failure of the oil pressure sender that stopped the lift pump. The engine continued to run, utilizing the head pressure of the raised fuel tank. Eventually the engine would not restart. I don’t know if we just ran the tank level down or the lift pump was needed for startup.

It appears like you may have a similar system. I was able to jumper the leads from the oil pressure sender and restart the lift pump.
Is it possible that your system is wired the same way with a bad oil pressure sender?
Note that it's prudent to ensure you have oil pressure prior to starting the engine. In our case we had a independant oil pressure gauge.

[hd002e]

This is extremely helpful. The wire from the glow plug switch goes to the dashboard with all the gauges and watning lights and I could not understand why that would be. So maybe thats where it integrates with the oil oressure switch. I will have to do a bit more digging around tomorrow! Thanks a lot for your help!

Originally Posted by Tsuru

Our last boat had a Universal M35B that used the same lift pump. It was wired to run all the time, but the leads ran through the oil pressure sender. If oil pressure was lost, power to the lift pump was cut. This was done to protect the engine from loss of oil pressure damage. Since the engine has no oil pressure prior to startup there was a bypass, which was activated when the glow plugs were engaged. To start you moved the key to glow plug position and the lift pump engaged. When the engine starts the oil pressure is restored and the lift pump continues to run.

At one point we had a failure of the oil pressure sender that stopped the lift pump. The engine continued to run, utilizing the head pressure of the raised fuel tank. Eventually the engine would not restart. I don’t know if we just ran the tank level down or the lift pump was needed for startup.

It appears like you may have a similar system. I was able to jumper the leads from the oil pressure sender and restart the lift pump.
Is it possible that your system is wired the same way with a bad oil pressure sender?
Note that it's prudent to ensure you have oil pressure prior to starting the engine. In our case we had a independant oil pressure gauge.

[tkeithlu]

Wow! I'd never heard of engines wired that way. My M25 genset is wired as your mechanic suggests: Glow plugs are a separate push button, throttle solenoid and lift pump are the "run" switch, and another push button is the starter. The temperature and pressure sensors set off alarms, so startup (no oil pressure) is a little noisy.

[hd002e]

So got everything put back together today. The engine starts running without the lift pump engaging by just gravity feed from the day tank. However after about 5 minutes, the RPMs drop and the engine dies. At that point I can run the lift pump by shorting the circuitry and the engine will spring back into life.

Is a lift pump supposed to be on at all times?

Is it at all possible that an engine can feed off a day tank just by gravity alone?

[nuku34]

In general, if the day tank is high enough above the engine, the engine should run without a lift pump. However, your Racor filter might be providing significant resistance to fuel flowing down from the day tank. Are you sure the filter is not constricting the flow? You can check this by disconnecting the outlet line from the Racor and see if you get a good flow of fuel by gravity from the day tank. There also could be a problem in the lift pump. Normally these electric lift pumps do not inhibit fuel flow when they are off, but that can also be checked by taking off the outlet hose and seeing if gravity from the day tank causes fuel to flow freely. There is nothing wrong with setting up the lift pump to run all the time that the engine is running, but that would make the gravity feed day tank redundant wouldn't it?