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Unread 08-18-2014, 06:58 PM   #1
cj7always
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1978 CJ7 
 
Join Date: Aug 2004
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Fuel pump losing prime

79 304
New lines, hoses etc. just put motor in and the fuel pump is losing prime from the tank if it sits very long. Any ideas why?
Just got motor in this past weekend and this morning had to put gas in carb to get it started and primed. It had been sitting at my father in laws since Saturday evening waiting to get my insurance and tags. It has new lines from carb back to tank, all rubber and steel lines replaced.

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Unread 08-18-2014, 07:40 PM   #2
OrangeCJ-5
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Mine would lose prime as well. After some research I found out the check valve is inside the pump. It eventually got to the point that it wouldn't run. Replaced the pump and all is well. Actually it runs better than it ever has. The pump must have been going bad for a while.
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Unread 08-18-2014, 08:12 PM   #3
LumpyGrits
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Bad pump.
Do confirm that ALL fuel line connections are TIGHT.
LG
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Unread 08-19-2014, 06:12 AM   #4
shadoow
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sounds exactly like what i had. Jeep sat for a few days & it would crank & crank & crank & finally fire up. Ran fine otherwise & if i started it every day it fired right up. Tried some things from posts on here (fuel filter alignment, tighten fuel connections, carb bowl leaking, etc.) but none changed anything. Finally bought a new fuel pump & problem solved.
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Unread 08-19-2014, 06:28 AM   #5
82JeepCJ7
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I had a similar issue, but found out the well plugs in the carb bowl were leaking, and would drain the fuel bowl over night. A simple carb rebuild as well as sealing the well plugs with JB Weld its been running fine ever since.

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Unread 08-19-2014, 04:46 PM   #6
Spieg8
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Quote:
Originally Posted by cj7always View Post
79 304
New lines, hoses etc. just put motor in and the fuel pump is losing prime from the tank if it sits very long. Any ideas why?
Your fuel naturally wants to return to the tank due to a combination of gravity and siphon action. For this to happen, air must get into the system to take up the space of the fuel as it flows back to the tank.

As has been mentioned, the most likely sources include loose fuel line connections or air being pulled in through the carburetor. Could also be a small hole in a hose, but that's usually easy to detect.

I'd start inspecting lines/connections at the tank and work my way forward to the motor. If that all passes inspection then it's probably time to start checking your pump or carb.

For about $5, you can also install an in-line back flow preventer in the fuel hose just before the pump... it's a band-aid, but it works.
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Unread Yesterday, 05:57 AM   #7
JeepHammer
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Quote:
Originally Posted by cj7always View Post
79 304
New lines, hoses etc. just put motor in and the fuel pump is losing prime from the tank if it sits very long. Any ideas why?
Just got motor in this past weekend and this morning had to put gas in carb to get it started and primed. It had been sitting at my father in laws since Saturday evening waiting to get my insurance and tags. It has new lines from carb back to tank, all rubber and steel lines replaced.
I just went through this myself again. Wasn't paying attention when I ordered a fuel pump and got a generic...
Crank the crap out of the vehicle to get it started, sputter, stutter, and generally barely ran.

1. Not all fuel pumps are created equal.
The 'China' or 'Mexico' import will not be the same quality as say, a Delphi pump. It's worth the hunt/wait for a Delphi.
Brass check ball valves, brass seats for those valves, ect.

Steel ball valves have a tendency to rust from moisture in the fuel.
That rusted steel ball valve beats the crap out of the soft seat material and pretty soon the seats are no longer fuel tight.

Stainless steel or bronze balls and brass seats won't have this issue, but are a few pennies more than steel... And some of the 'Discount' pumps actually don't have seats installed, they just surface the CAST aluminum for the ball valve to beat on...
And we all know how soft cast aluminum is!

2. Wrong gas cap and/or tank vent plugged.
When the vehicle cools down from operation, *IF* the tank vent is plugged and you don't have a vented cap, the fuel/air space will contract as it cools.
This creates vacuum and sucks the fuel right out of the line.

A very recent issue was the same, the guy had to use starting fluid to get the Jeep running the first time of the day.
If he didn't let the vehicle sit too long, it would start right back up.
Changed pump to a quality unit and it starts like it should.

3. Carb inlet valve BELOW fuel level.
If the carb inlet is ABOVE fuel level, like in a Holley, the line just sucks air from fuel bowl vent.
If your have an inlet BELOW fuel level, the line can suck the fuel right out of the carb float bowl!

Now, the guy with the Q-Jet had tube plugs leaking.
Not a lot of us have Q-Jets, but carbs CAN leak fuel into the intake when they sit still.
Some others can have internal leaks that will suck them dry, but lot's of others don't have this issue, just a fuel inlet below fuel level in the bowl.

Again, we are dealing with vehicles that are 3 decades old, and some leaks, squeaks, quirks, and failures are going to happen in that time!
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