The 1947 - Present Chevrolet & GMC Truck Message Board Network







Register or Log In To remove these advertisements.

Go Back   The 1947 - Present Chevrolet & GMC Truck Message Board Network > 47 - Current classic GM Trucks > The 1973 - 1987 Chevrolet & GMC Squarebody Pickups Message Board

Web 67-72chevytrucks.com


Reply
 
Thread Tools Display Modes
Old 03-01-2017, 10:46 AM   #1
m00n1957
Registered User
 
Join Date: Oct 2010
Location: blue mtn. ms
Posts: 100
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.
m00n1957 is offline   Reply With Quote
Old 03-03-2017, 12:27 AM   #2
El Dorado Jim
Registered User
 
Join Date: Dec 2009
Location: El Dorado Ca
Posts: 3,374
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
El Dorado Jim is offline   Reply With Quote
Old 03-03-2017, 12:08 PM   #3
Dead Parrot
Registered User
 
Join Date: Aug 2012
Location: Oklahoma City, OK
Posts: 2,611
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.
Dead Parrot is offline   Reply With Quote
Old 03-03-2017, 04:41 PM   #4
Tash4
Registered User
 
Tash4's Avatar
 
Join Date: Sep 2016
Location: Pinhook Mo
Posts: 120
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.
Tash4 is offline   Reply With Quote
Old 03-05-2017, 08:51 PM   #5
m00n1957
Registered User
 
Join Date: Oct 2010
Location: blue mtn. ms
Posts: 100
Re: fuel pump loses it's prime, gas runs back to tank.

Quote:
Originally Posted by Dead Parrot View Post
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.
doesn't matter what tank is selected, changed fuel pump again, still won't hold it's prime.
m00n1957 is offline   Reply With Quote
Old 03-05-2017, 10:36 PM   #6
cadillac_al
Registered User
 
Join Date: Oct 2013
Location: Maine
Posts: 2,376
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.
__________________
76 Chevy K20
76 GMC K15
77 Chevy C10
77 Chevy K10
cadillac_al is offline   Reply With Quote
Old 03-05-2017, 11:53 PM   #7
Alex V.
Registered User
 
Alex V.'s Avatar
 
Join Date: Mar 2013
Location: Campbellsville, KY
Posts: 888
Re: fuel pump loses it's prime, gas runs back to tank.

What cadillac al said was what came to my mind first.
__________________
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.
Alex V. is offline   Reply With Quote
Old 03-06-2017, 12:27 PM   #8
m00n1957
Registered User
 
Join Date: Oct 2010
Location: blue mtn. ms
Posts: 100
Re: fuel pump loses it's prime, gas runs back to tank.

Quote:
Originally Posted by cadillac_al View Post
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.
driver-side tank is full of gas, when i pressurize tank to get gas up to fuel pump i haven't found any gas leaking and i've put enough pressure that it will blow back on you if you release towel from gas inlet to quickly.
m00n1957 is offline   Reply With Quote
Old 03-06-2017, 04:43 PM   #9
Dead Parrot
Registered User
 
Join Date: Aug 2012
Location: Oklahoma City, OK
Posts: 2,611
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.
Dead Parrot is offline   Reply With Quote
Reply

Bookmarks


Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT -4. The time now is 09:01 AM.


Powered by vBulletin® Version 3.8.11
Copyright ©2000 - 2024, vBulletin Solutions Inc.
Copyright 1997-2022 67-72chevytrucks.com