NEW BUILD ISSUE WITH FUEL PUMP LOSING ITS PRIME

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J

johnnydemon

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Hello All,
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I have put about 150 miles on my new build, 1972 Dart Demon. During this time, I have changed the mechanical fuel pump three times due to fuel bowl becoming empty. Yesterday, I drove about 12 miles to a car show and while parking, again had 0 fuel pressure and no fuel in the fuel bowl. I let the car sit for three hours, primed the carb and the motor started right up. I drove the same distance back home and the same thing happened upon entering my garage.

I believe that I should now rule out the fuel pump.
Fuel filter near the fuel tank and another near the carb.
6 lbs of fuel pressure which holds steady until this situation occurs.
Metal 3/8" fuel line from the tank to about 12" from pump. 3/8" braided line from line to pump and then from pump to carb. The braided line DOES run from the pump through the alternator bracket and up to carb.
Lower radiator hose is about 3/6" away from fuel pump which I am speculating as a possible issue here.

The motor is a 408, 10.8:1 compression, roller cam, etc.

Any help or suggestions would be appreciated.
John
 
If I'm understanding this correctly, it sounds pretty normal. Pressure will only build if the needle and seat are closed with the engine running. Engine off, pump is off so no pressure. Car sits hot, gas vaporizes lowering the level of the bowls, so on start the pump fills them. The longer the car sits, the longer it will take to fill the bowls and start.

Did I miss the issue?
 
dano said:
If I'm understanding this correctly, it sounds pretty normal. Pressure will only build if the needle and seat are closed with the engine running. Engine off, pump is off so no pressure. Car sits hot, gas vaporizes lowering the level of the bowls, so on start the pump fills them. The longer the car sits, the longer it will take to fill the bowls and start.

Did I miss the issue?
Click to expand...
the bowl empties while the car is operating, not when sitting. So, while the engine is running, it eventually loses fuel pressure, bowl empties, engine shuts off. Must reprime the carb (or crank the motor for awhile) then the engine runs normal until the problem repeats itself, after about 12 miles of driving in 90 degree temps.
 
eekvonzipper said:
Mechanical Pump... Does it Really need a Filter in the Back? Maybe, try it w/o the rear filter just to see?
Click to expand...
You may have a point here. The rear filter is sized to capture larger particles while the one at carb is for finer. Its just odd that it takes 12 miles of drive time for the problem to occur.
 
What type of vent system on the fuel tank? Have you tried loosening the fuel cap when this occurs?
 

I do not know of an actual tank vent that I installed during build. Could you elaborate on where this went should be installed?

*I just looked at pickup installed in/on tank. There is a small nipple that has a rubber cap over it. Is this for a return or is this a place to vent? This is high on tank, above the sending unit/float assembly.

Thank you
 
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Is there any time the car does not exhibit the problem? Maybe when it's somewhat cool out?
 
RustyRatRod said:
Is there any time the car does not exhibit the problem? Maybe when it's somewhat cool out?
Click to expand...
Unfortunately, I didnt finish it until hot weather for the test drives. I can tell you that while breaking in/tuning while in my shop, it did it then. My shop is conditioned, so it was probably 74 to 75 in here.
 
johnnydemon said:
Unfortunately, I didnt finish it until hot weather for the test drives. I can tell you that while breaking in/tuning while in my shop, it did it then. My shop is conditioned, so it was probably 74 to 75 in here.
Click to expand...
Then unless you have a fuel line running close to something that gets hot, I think you can rule out vapor lock. That said, I've seen guys have trouble with vapor lock from the heat of the mechanical fuel pump. Are you running ethanol gas?
 
johnnydemon said:
the bowl empties while the car is operating, not when sitting. So, while the engine is running, it eventually loses fuel pressure, bowl empties, engine shuts off. Must reprime the carb (or crank the motor for awhile) then the engine runs normal until the problem repeats itself, after about 12 miles of driving in 90 degree temps.
Click to expand...
After running then shut down, the fuel evaporates due to heat. So they empty in gaseous form.
 
If you have any pressure drop on the INLET side of the pump it becomes very easy to vapor lock with a little bit of heat. Check the sock in the tank and the filter on the inlet side. And maybe ditch that filter. Make sure the lines aren’t collapsing or sucking air while the pump is running.
 
RustyRatRod said:
Then unless you have a fuel line running close to something that gets hot, I think you can rule out vapor lock. That said, I've seen guys have trouble with vapor lock from the heat of the mechanical fuel pump. Are you running ethanol gas?
Click to expand...
The fuel lines are not against anything hot, however, I really think the lower radiator hose is too close to the fuel pump (3/16") but that's factory. I am just running standard high test pump gas.
 
I am going to order the check valve and utilize the vent on top of the tank. My gas cap is non-vented so I am thinking that maybe this may correct things. I will report back in a few days to update. Of course its calling for 5 days of rain so it may be a minute...haha thanks for all of the help so far.
 
johnnydemon said:
I am going to order the check valve and utilize the vent on top of the tank. My gas cap is non-vented so I am thinking that maybe this may correct things. I will report back in a few days to update. Of course its calling for 5 days of rain so it may be a minute...haha thanks for all of the help so far.
Click to expand...
Or you could take the gas cap off and go for a drive. You’ll know if that’s the issue in about 12 minutes
 
Get the 72 service manual, over from MyMopar.com, and by the way guys on THIS board are why it's there. Read the fuel and emissions sections about the carbon can evap vent system. If you have eliminated the 1/4" tube that originally ran with the fuel line up to the front then YOU HAVE NO TANK VENT

PLEASE DO NOT try to use a vented cap, because sooner or later you will have FUEL DOWN THE SIDE of the car.

Download the 69/ earlier service manual and look how that vent is set up (simple) and works, and if you can fabricate a little bit, duplicate that system. It is simply a 1/4" tube into near the top of the filler tube, up and over near the top of the quarter panel to form a siphon break, then, ran down the side of the filler, through the floor gasket, and ends in the open rear frame rail.

Be advised that the vent system CHANGED almost every year after 70, and SO DID THE CAPS. A very knowledgable guy, Steve, OldManMopar on here has posted a photo previously, showing the different caps

NOTICE that Mopar caps ARE NOT VENTED in an operational manner. Pre evap controls AKA some 70's and earlier, are NOT vented and are SEALED. The evap controls caps have what is called "pressure vacuum" and these are spring pressure relief valves, both for pressure and vacuum, in case the thing builds too much pressure. No idea why yourse maybe does not

Here we go:

www.forabodiesonly.com

Duster fuel filler pipe difference between years

I would like to buy a new fuel filler pipe for my '72 Duster but can only find one for '70 / '71 with ECS or '73 / '74 with folding rear seat,or for '75 / '76. Can any of these be made to work on a '72? What changes were made between these years?
www.forabodiesonly.com www.forabodiesonly.com

abody-gas-caps-67-70-70-cal-and-71-72-74-75-76-copy-jpg.1715483422


I'm sure what you are doing is pulling a vacuum on the sealed tank, until the pump simply cannot pull anymore

Want to test it? Remove a goodly amount of fuel from the tank so that it' won't "slop" and then either remove the cap for a run or see if you can just loosen it.

Another thing you can do is drive it a ways, jump out and listen as you loosen the cap. A substantial "whoosh" of air will tell the story.

IF THE ORIGINAL 1/4" tube up to the front is still there and connected to the tank, it must come up high in the engine bay and be OPEN. Some guys tee them into the air filter bonnet vent to one valve cover.

That looks like a VERY nice Mopar
 
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You said that this also happened in your garage when doing the initial tuning, so you can duplicate the problem just sitting at idle or faster , right ??
Let's bypass the entire rear half of the fuel system.
Run a new section of fuel line from the inlet side of the fuel pump directly into a gas can filled with fresh fuel.
Try to duplicate the issue again.
This will eliminate possible air leaks in the system ( like sucking a straw with a hole between the level of the liquid and the intake side opening of the straw.)
or restrictions in the sending unit assembly.
Also pull a few spark plugs and check for signs of richness.
 
I loaned my pump out to a friend for some troubleshooting and it came back like : /

Second and third pic is the repair.

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20250707_070404.jpg


20250707_070359.jpg
 
The symptoms describe to me were identical to the op. Car would run fine however once it Heats soaked fuel pressure would go to zero. The installer blamed my fuel pump. I was insistent that he did not follow my instructions and was Vindicated upon inspection.
 
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