Fuel pump vacuum test

Finnerty

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Sep 20, 2010
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My 1977 Mercruiser 454 started running poorly at higher rpm, not going above 3000 rpm, stumbling with the occasional backfire. I tee'd a vacuum gauge into the fuel line before the pump and read 0.5 in Hg at 1800 rpm and between 0.5 to 1.0 at 3000 rpm with the gauge needle vibrating fast. I'm guessing I need a new fuel pump?
 

Bondo

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Re: Fuel pump vacuum test

My 1977 Mercruiser 454 started running poorly at higher rpm, not going above 3000 rpm, stumbling with the occasional backfire. I tee'd a vacuum gauge into the fuel line before the pump and read 0.5 in Hg at 1800 rpm and between 0.5 to 1.0 at 3000 rpm with the gauge needle vibrating fast. I'm guessing I need a new fuel pump?

Ayuh,.... I've always tested the fuel pump's Output, rather than it's suction effect...

The Output needs to be 'tween 4, 'n 6 psi...

Is the Tank freely givin' up it's fuel,..??
 

Finnerty

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Re: Fuel pump vacuum test

The tank is not restricting the fuel flow: it will siphon fuel through an open fuel line at rest (that's how I prime my fuel filter bowl after changing the element), and under high rpm I would expect the vacuum to rise quickly (it doesn't). I could've tried opening the fill cap if I thought of it at the time. It would be more difficult to put the gauge on the discharge side of the pump as the discharge fuel line is metal tubing. and hard to get at.
 

Failproof

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Re: Fuel pump vacuum test

Low vacuum when drawing fuel from the tank does not mean bad pump per se. It could just be easy flowing and not need much vacuum. At .5 in/mercury i am guessing no check valve. Usually takes 1.5 to 2 inches just to crack those. Best way to test vacuum ability of a pump is to dead head it. Vacuum gauge direct to pump, to see max. Vacuum. Then test output psi/flow. You already know the fuel is not restricted from point you had gauge to tanks, now to test from gauge connection to carb!
 

Don S

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Re: Fuel pump vacuum test

To test fuel pressure, make yourself a hose with a Tee fitting on it for the gauge. Use hose from the pump to the carb with the proper fittings on each end, and put the Tee in a convenient spot in that hose. Then test the pressure.

If you have a problem with fuel supply, check it like this. http://forums.iboats.com/showthread.php?t=167561

PS: You should be able to siphon fuel out of the tank, if you can, then the anti-siphon valve is either not working or been removed.
 

markheck1

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Mar 31, 2012
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Re: Fuel pump vacuum test

and Don is telling you you shouldn't be able to siphon fuel from the tank.
It's a HUGE safety issue, a leak can fill the bilge with fuel, and then you are sitting in an EXPLOSION waiting to happen.
 

Finnerty

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Re: Fuel pump vacuum test

Thanks, so I assume the anti-siphon valve is essentially a one-way valve I can plumb into my gas line somewhere?
 

Bondo

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Re: Fuel pump vacuum test

Thanks, so I assume the anti-siphon valve is essentially a one-way valve I can plumb into my gas line somewhere?

Ayuh,... It's the hose barb fittin', at the tank's outlet...
 
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