Boat dies at half tank of gas, fuel pump question

SeanT

Senior Chief Petty Officer
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All, having a minor issue with my boat.

When the boat gets down to about half tank of gas, it stalls and won't start again. It's happened a few times now. I've troubleshot it, and it appears that when this condition is occurring, there is no fuel exitting the pressure side line of the fuel filter. Okay, so either my fuel pump is failing or maybe I have a blockage. And right now I don't think it's a blockage, because if I fill my tank, everything runs fine like there's nothing wrong. See, my tank actually sits above the floor in the very back of my boat, and I think good old physics is on my side pushing fuel into the engine for me.

So anyway, I'm getting to the point here now - while troubleshooting this problem with my test light on my fuel pump I noticed that the pump is only getting power when I have the ignition switch at "start" (i.e turning the motor over), instead of just "on" (i.e the first click). Do you think this is normal? Is the pump only supposed to operate while I'm turning over the engine?
 

SeanT

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Re: Boat dies at half tank of gas, fuel pump question

Oops - I may have put this in the wrong section since this is engine related. Apologies!
 

a70eliminator

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Re: Boat dies at half tank of gas, fuel pump question

Almost guarentee it's not the fuel pump, theres's probably something going on with the pick-up tube, it should go to the very bottom of the tank, don't know if yours a rigid piece of pipe or a rubber tube you may need to remove the tank.
 

nlain

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Re: Boat dies at half tank of gas, fuel pump question

If it happens at half tank every time then check for a cracked pickup tube. I could have a crack or small hole in it right at the half tank area, the pump will pull air easier than it will pull fuel.
 

SeanT

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Re: Boat dies at half tank of gas, fuel pump question

I'm going to see if I can check that right now. If I pull the fuel line off the input to my fuel filter, I shouldn't get any fuel out of it if that's the case (in fact, I should be able to blow back through it).

In the meantime, does the fuel pump behaviour I mentioned above sound right? Should it only get power when I turn the key to start the engine?
 

SeanT

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Re: Boat dies at half tank of gas, fuel pump question

I've pulled of the fuel line that runs to my filter. I couldn't blow into the tank because there's probably a check valve (which my dad reminded me about as well). However, I can suck fuel out of that line, which tells me that it probably isn't my pick up tube.
 

nlain

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Re: Boat dies at half tank of gas, fuel pump question

Ok, now check the rest of the line to the pump, especially if it is rubber, there has to be something causing it not to pull fuel after half tank and I do not think it is the fuel pump causing this.
 

SeanT

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Re: Boat dies at half tank of gas, fuel pump question

Couldn't the "weight" of the gas in the full tank force the line, then as the tank level drops gas no longer flows? My tank sits above the fuel filter and pump in my boat.

The only other thing it could be is a completely clogged fuel filter, but then I shouldn't have fuel flow when the tank is full either.
 

a70eliminator

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Re: Boat dies at half tank of gas, fuel pump question

I suppose that could be happening, maybe you do have a bad fuel pump? maybe try to pump fuel into a container right off the pumps outlet take it off the carburtetor and into a jug, then apply voltage to the pump.
 

rbh

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Re: Boat dies at half tank of gas, fuel pump question

If it suck's the 1st half, it is not the pump.
 

SeanT

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Re: Boat dies at half tank of gas, fuel pump question

If it suck's the 1st half, it is not the pump.
My suggestion is that the pump is not sucking at all. Gravity is doing it until the tank level equalizes with the engine. This is a direct drive boat with the tank in the back, above the floor, higher than half the engine.
 

rbh

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Re: Boat dies at half tank of gas, fuel pump question

Sean, My Alberta neighbour-
I do not think that a motor could run very well with just a siphon charge???
You might be suckin air at that point.
 

Fisherball

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Re: Boat dies at half tank of gas, fuel pump question

Could it be a clogged vent? The pump has the power to 1/2 empty the tank but after that there is too much vacuum for it to overcome? Maybe a dumb question.
 

SeanT

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Re: Boat dies at half tank of gas, fuel pump question

Not sure. I can tell you that I was able to pull off the fuel line running into my fuel filter and suck gas out myself, so I doubt that's it.

I'll know more tomorrow, but if anyone else has had this, please let me know.

I suspect the oil pressure relay switch right now as well, which apparently fires the pump up. I think I need to try putting 12V on the pump to make sure it's actually working. That'll be the next thing I try, I've overlooked it so far.
 

cr2k

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Re: Boat dies at half tank of gas, fuel pump question

Maybe the bottom half of the tank is water.
 

ENSIGN

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Re: Boat dies at half tank of gas, fuel pump question

Remove the dip tube from the tank there's a crack in it and a visual inspection is the only way to tell.
 

SeanT

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Re: Boat dies at half tank of gas, fuel pump question

Looks like I may have had an intermittent connection to my fuel pump. I've read about this on the Moomba forums.

If I unhook the pressure side line of the pump from the fuel line to the carb, then apply 12 volts, it shoots gas. I didn't put a gage on it, but it showed that it wasn't completely failed like I thought.

Put the connection back on, wouldn't start. Moreover, it wasn't even kicking in. Pulled the connection off, cleaned it out and tightened it, shoved it back on and everything was running and starting fine. Still staying in the bay today in case I have a reoccurrence (I have tow support lined up if I need it).

Thanks for the replies all, if I have anything to add, I'll update the thread.
 

rbh

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Re: Boat dies at half tank of gas, fuel pump question

Glad to hear you got it figured out, It threw me for alittle loop when you said it ran good on the first 1/2 of the tank.
Live and learn I guess. :)
 

SeanT

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Re: Boat dies at half tank of gas, fuel pump question

Glad to hear you got it figured out, It threw me for alittle loop when you said it ran good on the first 1/2 of the tank.
Live and learn I guess. :)
You and me both!

It is definitely odd that it _seemed_ to happen at half tank of gas, and I'm still cautious about the issue even though it appears to be resolved. The previous comment about whether an engine can run on a siphon charge alone - I'm thinking maybe. I've been told that a 4 barrell carb goes through 2 cups of fuel in a minute. I know I can siphon at least that quickly.
 
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