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03-01-2017, 10:46 AM | #1 |
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Join Date: Oct 2010
Location: blue mtn. ms
Posts: 100
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fuel pump loses it's prime, gas runs back to tank.
75 gmc 3/4 ton, dual tanks, 350, 1406 edelbrock carb. only way to get gas to fuel pump is pressurize the tank, runs good till shut off and gas drains back, have clear fuel filter and soon as shut off it empties. i've change fuel pump and tank selector. i've blown thru all lines, all are clear. doesn't matter if gas cap is on or off. no leaks. thanks in advance.
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03-03-2017, 12:27 AM | #2 |
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Join Date: Dec 2009
Location: El Dorado Ca
Posts: 3,374
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Re: fuel pump loses it's prime, gas runs back to tank.
I think the mechanical fuel pumps have a check valve, maybe it is bad
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03-03-2017, 12:08 PM | #3 |
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Join Date: Aug 2012
Location: Oklahoma City, OK
Posts: 2,611
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Re: fuel pump loses it's prime, gas runs back to tank.
Does yours have a return line?
Does it work the same regardless of which tank is selected? Does it start ok or take a lot of cranking to start? I have noticed that those clear fuel filters often seem empty even if the engine is running and everything working fine. |
03-03-2017, 04:41 PM | #4 |
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Join Date: Sep 2016
Location: Pinhook Mo
Posts: 120
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Re: fuel pump loses it's prime, gas runs back to tank.
new fuel pump fixed this problem on my dads 85. truck had set for a year or so, still started and ran but would loose prime after a day.
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03-05-2017, 08:51 PM | #5 | |
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Join Date: Oct 2010
Location: blue mtn. ms
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Re: fuel pump loses it's prime, gas runs back to tank.
Quote:
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03-05-2017, 10:36 PM | #6 |
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Join Date: Oct 2013
Location: Maine
Posts: 2,376
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Re: fuel pump loses it's prime, gas runs back to tank.
It must be a pin hole in one of your lines. I found a pin hole on my K20 once by using a Mityvac to pull gas from the fuel tank. I could hear it hissing from under the hood. They may not all be that loud but you should be able to hear it. I just fixed a pinhole in my Caddy and the car starts so much easier now. It was a bear to start for the last few years (same with the K20). I found that leak once it started dripping.
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76 Chevy K20 76 GMC K15 77 Chevy C10 77 Chevy K10 |
03-05-2017, 11:53 PM | #7 |
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Join Date: Mar 2013
Location: Campbellsville, KY
Posts: 888
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Re: fuel pump loses it's prime, gas runs back to tank.
What cadillac al said was what came to my mind first.
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Alex V. ------ 1967 C10 Suburban, 350/NP435, Green/Green, PS, PB, HD cooling, charging, shocks, and springs. 1985 GMC C3500 SRW, Sierra Classic, 454/TH400, white/blue. |
03-06-2017, 12:27 PM | #8 | |
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Join Date: Oct 2010
Location: blue mtn. ms
Posts: 100
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Re: fuel pump loses it's prime, gas runs back to tank.
Quote:
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03-06-2017, 04:43 PM | #9 |
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Join Date: Aug 2012
Location: Oklahoma City, OK
Posts: 2,611
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Re: fuel pump loses it's prime, gas runs back to tank.
Is your problem just the issue of fuel not remaining visible in the filter when the motor is off? Does the truck start quickly or need a bunch of cranking to start?
On my 75 with dual tanks, I normally run one tank completely empty to the point the engines stumbles then switch to the other side. This means the pump and line from tank valve to the pump is empty of gas yet within a second or so after switching to the full tank, everything is running normally. This behavior seems to indicate that the fuel pump doesn't need a 'prime' to suck gas from a tank with gas in it. |
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