72 Demon starts bogging sporadically once warm

Fuel and Air Systems

  1. Markzilla88

    Markzilla88 Active Member

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    Hey all, I’m pretty new around here, my wife and I just picked up a 72 demon that the P.O. set up for the track and street. It has a mild 360 with a lunati bracket master II .480 cam in it, Edelbrock performer intake and headers. 727 trans with a 4,500 stall. 8 3/4” rear end with 3:91 sure grip (I believe from dr diff). It has an Edelbrock 17301 electric fuel pump. I took it out 3 times yesterday, first two trips were short around town rides maybe a total of 5 miles each. Temp stayed at 180 the whole time and it ran great no issues. My third and final trip was alittle longer, maybe just a few more miles. I was cruising at about 40 mph going down a mild grade hill when I noticed the car started to bog down. I didn’t change the accelerator pedals position, it just started to bog down. I kept it in its same place and it cleared up and regained full power a few seconds later. It ran okay for about 15 seconds and bogged down again in the same fashion. I pulled into my destination which was right immediately after this happened. Let it sit for about an hour. It fired right back up, didn’t seem to have any issues. Temp was around 150. I stared to climb a decent grade hill immediately leaving the parking lot going about 30 mph. It started bogging again. This time it ran full power, then bogged for a good 20 seconds, cleared up, then bogged again for longer. I got out of the pedal and it cleared up, only momentarily this time. I tried to gain speed again and it gave me power but only for a second or two then bogged really bad. It got to the point where I would get off of the pedal it would idle, then I’d try to gas it and it’d bog. Repeat this for about a minute with each bog getting worse. It then died on me. I coasted it off the road, called for a tow and waited. After waiting about 10 mins I restarted it. It fired fine. It fully revved while in park and neutral, and never bogged or hesitated. I checked the fuel pressure and it was sitting at 5 psi. After letting it idle for a minute it dropped to 2 1/2 - 3 lbs. If i gassed it it would drop slightly lower.

    That’s the big picture of what happened yesterday. Does anyone have any idea what happened or what it may sound like??

    Today I changed the inline fuel filter under the hood and re-grounded the fuel pump ensuring it was grounded. Ran a fuel pressure test again. Cold start it sat at 7 1/2 psi. Once warm to 170 degrees it dropped to 4.5 - 5 psi. I held the throttle in just a tad (unsure what rpm as I was in the engine compartment not the cab) and it dropped to 2 1/2 psi. When I let it go to idle again it was a slow climb back to 4.5 - 5 psi. The fuel filter also look low in gas but then started to fill back up to about 1/4 full slowly.

    I know that’s a book, but any help is greatly appreciated! Thanks
     
  2. mderoy340

    mderoy340 Well-Known Member

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    Fuel pump is probably going. Check the fuel bowls for crap inside, if clean you have a fuel delivery problem. Fuel pump or pressure regulator.
     
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    • missing linc

      missing linc Loose nut behind the wheel FABO Gold Member

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      Plugged filter maybe, or fuel pump on it's way out, does it have a fuel pressure regulator?
       
    • AJ/FormS

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

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      My guess;fuel-tank vent not working. or pump sucking air
       
    • Markzilla88

      Markzilla88 Active Member

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      So it has a regulator on it that appears to be new, same with the pump and tank honestly. After crawling under it last night though I noticed on the tank that the P.O. messed with the tank. It has 4 vent tubes on the side. 3 of them are pinched closed and the one left open has a 4” piece of hose pointing to the ground (dumps fuel out when tank is full). Then Coming off the sending unit, there’s two nipples. One has the fuel line attached to it going to the pump mounted up on the frame, but the other one has a short piece of hose that has a bolt in the end of it to block it off......

      Anyone have any idea the correct purpose of all those blocked vents or a diagram of what they SHOULD look like when properly used? Or any idea if that’s the culprit? Thanks again
       
    • Markzilla88

      Markzilla88 Active Member

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      I also replaced the inline filter under the hood with a clear one so I can see if it’s getting fuel or not, which it is getting fuel, but after the motors warmed up and you hold throttle for some time the level falls, but has a hard time refilling.
       
    • Markzilla88

      Markzilla88 Active Member

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      So I regrounded the fuel pump wire, installed a new fuel filter, removed all the old fuses and exchanged them for new fuses and cleaned the terminals and then changed the oil for good measure. I took it out for a ride again today and same thing, once I was trying to climb a hill it started to bog on me... I turned around immediately and went home under bogging power. I immediately opened the hood while the engine was running and checked the fuel pressure and it read 0 psi.... Can anyone explain why this may be happening (other then just a broken pump)? The fuel pump is running the whole time, I don’t understand. I can rev the engine just fine sitting in park or neutral, but the second i try to drive under load up a hill it seems to bog and want to die...
      Thanks in advance
       
    • missing linc

      missing linc Loose nut behind the wheel FABO Gold Member

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      Sounds like AJ was on the right track, and you may have a venting issue. I am not familiar with the evaporative emissions system on that style of tank, but clearly the hose dumping out fuel is not a good idea. 0 psi fuel pressure obviously is a problem too, can you adjust the regulator? The only other thing I can think of at the moment is the fuel pump is not getting a full 12 volts.
       
    • AJ/FormS

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

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      Is it possible;
      that when the car is not level the fuel pump sucks air at or in the tank. Since it runs continuously, the pump tries to fill the line with air and the pressure falls to zero. The engine still runs on whatever is in the bowls. When the car returns to being level, the pump once again fills the line with fuel and away you go.
      But if the engine stalls before the line is filled,and you turn off the key, which turns off the pump; then there you sit with an empty filter.If this problem is worse as the fuel tank empties, then I think this is a possibility.
      I have also seen it happen that the jumper to the pump from the sender sucks air under the clamp, and this also gets worse as the tank empties. When using gear-clamps, I always use two per end and stagger the screw heads as close to 180* as possible, cuz screw clamps tend to bunch up the hose at the screw heads, and do not properly grip the hose at the screwhead. It may look tight, but it almost never is. The factory spring-clamps work best.
      But if the jumper is proven to be sealing, then I would drop the tank, remove the sender, and inspect the pick-up.
       
    • mderoy340

      mderoy340 Well-Known Member

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      The pump you are using needs to be gravity feed to work properly. It is not meant to pull fuel up and out of a tank. It is also way to small for your engine and is meant for lawn and garden equipment. Just because something looks new does not mean it is working properly. Very few companies have good quality control today.
      Edelbrock Universal Micro Electric Fuel Pumps 17301
       
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      • Demonic

        Demonic Well-Known Member

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        Sound like you're missing the vapor-liquid separator. I thought the last year was '71, maybe yours is an early build. The four connection vent goes with it. Should be on a bracket in the corner of the trunk. You don't need it to get around your problem, though.

        ECS.gif
         
      • Markzilla88

        Markzilla88 Active Member

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        Thank you guys so much for the help, can’t tell ya how much I appreciate it. I think my plan moving forwards is I need to get a new and better electric fuel pump that can pull fuel and isn’t gravity fed, and then also I think I should either get a new tank or figure out how to properly vent my current tank. (My only concern using the same tank is that he crushed 3 of the 4 vent tubes...)
         
      • Markzilla88

        Markzilla88 Active Member

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        I know I can adjust the regulator, just don’t know if there’s a setting on it where I can adjust idle pressure and then load pressure if you will. I assume there’s not though. Again I’m thinking it’s fine holding pressure at idle on relatively flat ground but once you travel over half a mile up and down some hills there’s no pressure.

        It’s all starting to make sense now though... he set it up for the quarter mile in a rush. When I asked him do you drive it around town he said yes sometimes on the weekends, assuming now he meant very short trips. Theres really no recourse at this point for me, so I’ve just gotta keep moving forwards towards a solution.
         
      • missing linc

        missing linc Loose nut behind the wheel FABO Gold Member

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        Hopefully a new pump resolves the issue, when an engine is set up for 1/8th or 1/4 mile racing only, there is little thought or need to make it reliable for any distance. The car I have been working on this year was set up for 1/8th mile, has required several major changes and more to come. A little patience goes a long way to gaining experience. Keep us posted.
         
      • AJ/FormS

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

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        As you can see from the drawing, with the separator gone there is no other choice but capping or plugging.One vent should be enough.IIRC that 4-corner system was only a 1 or 2 year deal. I thought it was only found on E-bodies, but I guess I was wrong.
        That line going to the front, dips down somewhere close to under the rear seat, and I have seen it rusted solidly shut there,from the inside, and so, not venting at all. Mine was only a 3/16 line
         
        Last edited: Oct 13, 2019
      • Markzilla88

        Markzilla88 Active Member

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        I’ve gotta do something to reroute that line to stop it from just draining fuel out though. I mean it’s a steady flow dumping out when I fill the car up.. I’m thinking running the line up to equal level as the filler neck somewhere to stop it from draining.
        I’ll have to do some research on that other line coming out of the sending unit though to find it’s purpose.
         
      • Markzilla88

        Markzilla88 Active Member

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        I just thought of this.... there’s a stock mechanical fuel pump still mounted on the block. It looks almost new. I’m half tempted to reroute the fuel line from the tank to bypass the electric pump and run it to the mechanical pump. Just to see if it’ll keep good pressure while running it
         
      • AJ/FormS

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

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        Ou that's not good.
        My Barracuda has exactly that; a steel line from the sender runs alongside the filler tube and re-enters the tube on the top,about 4 to 6 inches from the quarterskin. It's all on a continuous grade so liquid entering it during fill-up should drain back down. But occasionally I see something sputtering into the filler pipe, during fueling..
         
      • missing linc

        missing linc Loose nut behind the wheel FABO Gold Member

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        I would at least try the mechanical pump, what have you got to lose but a little time?
        The other line from the sending unit is possibly a return from the engine compartment, there may have been a charcoal canister as part of the evap system, probably long removed by a previous owner.
         
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        • AVALIANTEFFORT

          AVALIANTEFFORT 71 Valiant Pro Street

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          I’m having a similar issue with a track built car. QFT 300 and when it gets hot it boils the fuel and pressure drops. I have a thread out about this and plan to run a mechanical pump as primary with the QFT in the switch as a boost option when racing.
           
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