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Fuel pump blowing not sucking!

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Old 03-22-2014, 10:45 PM
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Fuel pump blowing not sucking!

Recently replaced my fuel pump on my f250 with a 400. Diagnosing a no fuel problem and hooked my fuel pump vaccum gauge thingy up and seen that its making vaccum at the output side, and pressure at the input.... Took the new one out and simply pressed down on the arm...yup blows air out of the input side. Got the old one...sucked my finger right to it with one press of the lever. What would cause a brand new fuel pump to do this? All I can think of is a broken diaphram or something?
Thanks
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Old 03-22-2014, 10:53 PM
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Wrong fuel pump perhaps? Or, built incorrectly?

The only engines that this fuel pump goes on is a 351M/400; they're pretty unique. But maybe there's some unique difference between pumps built for cars and pumps built for trucks in terms of inlet and outlet orientation. That seems like a stretch though.
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Old 03-23-2014, 12:47 AM
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Originally Posted by fmc400 View Post
Wrong fuel pump perhaps? Or, built incorrectly?

The only engines that this fuel pump goes on is a 351M/400; they're pretty unique. But maybe there's some unique difference between pumps built for cars and pumps built for trucks in terms of inlet and outlet orientation. That seems like a stretch though.
I forgot to mention that the fuel pump worked fine for a period of 3 weeks for maybe 1 - 2 hours of total run time. The pump's exactly the same, one side (inlet) has a nipple that connects to the fuel line, the other (outlet) has a threaded nut that has a steel line that runs up to the carburetor so I can't see the ports being connected backwards.
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Old 03-23-2014, 02:20 AM
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Sounds like the new one internally broke to me, if did work and now doesn't. Sounds like the old one might need to go back on, what made you change it in the first place?
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Old 03-23-2014, 03:36 AM
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Well if that's the case, how did it work for 3 weeks? It's not like the arm jumped above the cam??
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Old 03-23-2014, 05:32 AM
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hypathetically, the fuel pump only needs a pair of checkvalves and a diaphram to pump fuel, I've never actually opened one up but it's wierd that both the input and output check valve would reverse themselves.
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Old 03-23-2014, 07:23 AM
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I don't think it's physically possible to install the pump arm above the eccentric.
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Old 03-23-2014, 07:31 AM
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"I think" ??? you can get it (the arm) above it on the initial install into the block...cause I am sure I have done it. lol

But you can not get it bolted down flush due to the tension. Sound like a internal failure of the pump.
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Old 03-23-2014, 08:04 AM
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Quote:
Originally Posted by Buyerz View Post
Recently replaced my fuel pump on my f250 with a 400. Diagnosing a no fuel problem and hooked my fuel pump vaccum gauge thingy up and seen that its making vaccum at the output side, and pressure at the input.... Took the new one out and simply pressed down on the arm...yup blows air out of the input side. Got the old one...sucked my finger right to it with one press of the lever. What would cause a brand new fuel pump to do this? All I can think of is a broken diaphram or something?
Thanks
Sounds like a defective inlet check valve. If the tank is full, the pump could manage to supply enough fuel to keep the carb full. With only 1-2 hours of run time, it might not show up or the valve could have failed during that time.

Check the hose on the inlet as well. The inside of the hose can delaminate and act like a valve at highway speeds.
jim
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Old 03-23-2014, 10:57 AM
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The fuel pump was installed correctly, has a perfect shiny spot right where it should be. I'm putting my money on the check valve. It was a cheapy from lmc truck. Lesson learned I suppose!
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