Fuel pressure problem..I'm at a loss.

Fuel and Air Systems

  1. plymouth67

    plymouth67 It be a lot cooler if you did...

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    The 512 in my Challenger has a issue with losing all fuel pressure. Let me start by stating that everything in the car is new...pump,lines all of it.

    This happens about an 1/8 mile after I stand on the gas, runs fine while throttle is down and then, about 1/8 mile down the road after I let off it starts bucking and, eventually will die with ZERO fuel pressure. Let it sit for 5 minutes and its good to go until I stand on it again.

    I can drive this car for hours and have no problems at all until I get down on it.

    I also want to include that I had this kind of issue last year but, when it was chugging before it would die it was actually blowing gas out of the vent in the front bowl, I replaced the float and it does not do that anymore. I also put a new Carter mechanical pump on this spring and right out of the box it was pushing 9.5 lbs of fuel pressure so, I put a Holley regulator on it and, dialed it down to around 5.5 lbs. I had also found some debris in the regulator one when this happened and, found that the sock on the sending unit had came off in the tank so, I also replaced the sock and, while the tank was out we blew air through the fuel lines to get any debris out of the lines, really thought that was going to solve it...it did not!!!

    I have noticed that fuel pressure seams to drop after the engine has warmed up. When I first start the engine it will have 5.5 to 6 psi but after its warm I have seen it all the way down to around 4 psi. So at 4 psi if I drive it responsibly I figure its enough psi to run it but not enough to feed the 850 double pumper with 50cc accelorator pumps? Why the big drop in fuel pressure once it warms up? I'm at a loss...
     
  2. 67Dart273

    67Dart273 FABO Gold Member FABO Gold Member

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    I am biased. I dislike front mount mechanical pumps. They amplify any vapor lock proplems because the entire supply path from the tank is under vacuum, therefore boils more easily, 'specially with today's alky polluted fuel

    Connector hose sucking shut? Restriction in tank pickup? Pump has a valve problem AKA dirt in one of the pump valves? More debri in regulator? A general restriction anywhere in the line?

    Re-re-re check the tank vent system..............................

    "After it warms up" might indicate vapor lock/ fuel boiling problem.

    If you know what kind of power the thing makes, calculate what full throttle fuel useage should be and do a pump VOLUME test. this is best done by making a restriction/ orifice at the line outlet to maintain fuel pressure, so the pump has to see it's normal working head
     
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    • plymouth67

      plymouth67 It be a lot cooler if you did...

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      I have been trying not to put an electric pump on it because I just don't want to hear it but, I think its pointing in this direction. The engine makes 581hp and 686ft lbs.
       
    • Dana67Dart

      Dana67Dart The parts you don't add don't cause you no trouble FABO Gold Member

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      If you can get alcohol free fuel in your area run your tank down to almost empty fill it up with alcohol free, drive it down and refill again then go testing.

      My stock 67 273 2bbl would do the same thing after ideling in a drive through line in the summer. Now with alcohol free doesn't do it anymore.

      If you let the car idel after it is hot, does your idel speed start to decline, then all of a sudden start to return? Maybe 1 minute between cycles.

      This what I encountered, just ideling.
      Imagine the filter being empty as you punch it, the bowels are still full enough to run the car for a few seconds and at the point the float gets low enough to allow new fuel into the bowl, it has to vent all the fuel vapors first and then refill the filter and bowl.

      https://photos.app.goo.gl/S5t4cX7FXFmRZhSS6
       
      Last edited: Jul 18, 2021
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      • plymouth67

        plymouth67 It be a lot cooler if you did...

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        The idle doesn't change, it's always real steady. With an electric pump this would all go away I think
         
      • TT5.9mag

        TT5.9mag Two atmospheres are better than one

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        With that kind of power you need a nice “engineered” well thought out system to deliver enough fuel to the engine at WOT, AND return unused fuel to the tank at idle. FYI there are in tank electric pumps that are quiet and you cannot hear them at all with the engine running.
         
      • pishta

        pishta I know I'm right....

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        ditch the regulator. Jamb a holley jet into your fuel line before the carb and see what pressure you get after this. I think your regulator is faulty if your pressure goes to zero, ie its at full passthrough on a bypass or its on full lockdown on a deadhead.
        ?u=https%3A%2F%2Ftse2.mm.bing.net%2Fth%3Fid%3DOIP.jpg
         
      • plymouth67

        plymouth67 It be a lot cooler if you did...

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        Do they make them to fit in the stock tank? Stock location and still uses the factory gauge?
         
      • plymouth67

        plymouth67 It be a lot cooler if you did...

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        I would tend to agree but, it did this before the regulator was on it also.
         
      • pishta

        pishta I know I'm right....

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        sucking the sock flat? fuel line air leak? can you put a vacuum guage on the pumps fuel inlet and run the motor off another fuel source to see if the check valves are not hanging up? the turret style is serviceable , IIRC there are 3 check valves in those, 2+1 in different directions. I ran across one that had a check valve that had come out (pressed in only) and was floating around in there. Did not produce any useful pressure.
         
      • pishta

        pishta I know I'm right....

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        someone did...skinny one that slides right in the sender hole. Of course I cant find it now...small walbro pump that you strapped to your old sock mount and somehow ran the wires out your sender ... or cut a hole in your OEM tank and run this
        ?u=https%3A%2F%2Fcdn.performanceonline.com%2Fimages%2FD%2Fprd_zm_84.jpg
         
      • TT5.9mag

        TT5.9mag Two atmospheres are better than one

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        Tanks inc does and aeromotive does. There may be others. Holley I believe makes a drop in sending unit using a stock tank.
         
      • Dana67Dart

        Dana67Dart The parts you don't add don't cause you no trouble FABO Gold Member

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        IMHO.

        before changing a bunch of stuff get an external electric (if that's your direction) and test it out to be sure it fixes your issue.

        As mentioned earlier, be sure you have enough venting to the tank. Might even do some testing without the fuel cap on, just watch the corners.
         
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        • plymouth67

          plymouth67 It be a lot cooler if you did...

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          My tank has the 4 vent lines on it.
           
        • pishta

          pishta I know I'm right....

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          Dissect that pump! the suspense is killing us....You wont ruin any gaskets.
           
        • Dana67Dart

          Dana67Dart The parts you don't add don't cause you no trouble FABO Gold Member

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          But are they all open to the atmosphere?
           
        • 67Dart273

          67Dart273 FABO Gold Member FABO Gold Member

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          Exactly. Try making a run with a makeshift cap that you KNOW is vented

          All 4 of those lines unless modified went into one which came up front for the carbon can setup. If that has been modified/ plugged, "there might not be a vent"
           
        • plymouth67

          plymouth67 It be a lot cooler if you did...

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          This is in an ebody so the 4 lines come from the tank and run up through the trunk floor into a vapor- liquid seporator. There is a 5th line from the seporator that should go to the breather in the valve cover but, its just open to the atmosphere. I can say the return line from the tank is not hooked up in the engine bay, it just has a cap on it. Also on this type system in an ebody the cap should not be vented.

          20190811_143258.jpg

          20210718_164932.jpg
           
        • 67Dart273

          67Dart273 FABO Gold Member FABO Gold Member

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          Where does this open line terminate? It should not be into the trunk!!! And that would be your vent right there
           
        • plymouth67

          plymouth67 It be a lot cooler if you did...

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          It terminates under the trunk floor
           
        • Dana67Dart

          Dana67Dart The parts you don't add don't cause you no trouble FABO Gold Member

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          Maybe the separator is clogged or not hooked up correctly?

          easiest thing to try is straight line hard pull no gas cap on Of it doesn't do it, it is a vent issue.
           
        • AJ/FormS

          AJ/FormS 68 B'cuda fb, Form S clone ... 367/A833/3.55s

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          what if the holes in your float valves are too small to supply your engine at WOT, and after a couple of seconds , the bowls run dry. The fuel pressure will drop but not to zero, as the valves are still a restriction.
           
        • plymouth67

          plymouth67 It be a lot cooler if you did...

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          After WOT the pressure drops to zero
           
        • plymouth67

          plymouth67 It be a lot cooler if you did...

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          I have checked the seporator and it's free and clear and, hooked up correctly.
           
        • plymouth67

          plymouth67 It be a lot cooler if you did...

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          I really feel like this is a sort of vapor lock situation. The return line from the engine compartment back to the tank is not hooked up and the regulator I have is not set up for a return line. I have ordered a regulator with a return and will try that first. If I wind up going with an electric fuel pump I will need this style regulator anyway.
           
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