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Fuel Pump Return Line

Discussion in 'Early CJ-5 and CJ-6 Tech' started by Dale Boyd, Oct 10, 2019.

  1. Oct 10, 2019
    Dale Boyd

    Dale Boyd New Member

    Pekin, Indiana
    Joined:
    Jan 8, 2018
    Messages:
    31
    After installing an internally regulated alternator (Powermaster 100/70 - works great) I took it for a test drive to make sure everything was back together and working. While driving everything looked good electrical-wise, but when I shut it down and got out I noticed fuel dripping from a splice in the fuel line. I replaced the hose coupling, but after tightening the clamps it was still leaking. I finally figured out the hose was so old it was cracking as I tightened the hose clamps. I ran new fuel line, but as I was attaching one end to the fuel pump I noticed something odd.


    Fuel Pump.jpg
    As I understand it, the second rubber hose is the return line that is supposed to run back to the tank. In this case, the return line coming out of the pump has been capped off. The hose from the tank is fastened to the capped return hose, but it is not tied in to the flow path. What at first looked like a "T" connection is actually just the return line clamped on to the side of the capped line. The return line is essentially a long open fuel tank vent.

    Is there any reason why the return line would have been capped off like this? Should I reconnect the return line to the tank, or just leave it alone? It runs fine like this.
     
  2. Oct 10, 2019
    jzeber

    jzeber Well-Known Member Sponsor 2019 Sponsor

    Morgan Hill, Ca
    Joined:
    Nov 15, 2003
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    2,391
    I have the same thing on my 231. The return is also plugged and it runs fine.
     
    jpflat2a likes this.
  3. Oct 10, 2019
    Norcal69

    Norcal69 Out of the box thinker 2019 Sponsor

    Cottonwood, Ca
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    Sep 6, 2014
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    1,155
    Mine is the same way. Been that way for the last 10-12K miles and in 100+ degree temps with no issue. I am running dual tanks so the return would be a pita.
     
  4. Oct 11, 2019
    mortten

    mortten I can’t put my finger on it

    Peninsula, Ohio
    Joined:
    Apr 17, 2006
    Messages:
    1,238
    I had mine capped and had vapor lock issues. Ran a return to the tank and solved the problem.
     
  5. Oct 17, 2019 at 8:06 PM
    Dale Boyd

    Dale Boyd New Member

    Pekin, Indiana
    Joined:
    Jan 8, 2018
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    31
    So after getting everything put back together I took it on a test run to the nearest town for about a 30 mile round trip. It ran great and all of the gauges (except fuel) are working now. One odd thing I did notice is that on a level section of highway the speedometer needle is steady. When I get to a hill and give it some gas, the needle starts bouncing +/- 5 mph or so. Anyone have an idea why the speedometer might get squirrely when the engine is under a load?
     
  6. Oct 17, 2019 at 9:08 PM
    n6ifp

    n6ifp Member Sponsor 2019 Sponsor

    San Bruno/...
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    Sep 15, 2004
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    206
    mine has been capped for years, runs fine.
     
  7. Oct 18, 2019 at 9:24 AM
    timgr

    timgr Jeepin' Nerd Sponsor 2019 Sponsor

    Medford Mass USA
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    Aug 10, 2003
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    20,102
    The fuel return is really simple. The type that puts the return in the fuel filter is just a small orifice that allows a limited amount of fuel to pass through and return to the tank. I presume the fuel pump type is similar. If that orifice is plugged (as when the return is capped here), pressure in the fuel pump builds to a fixed value (say 3 to 5 psi) and stops. When the float valve opens, some fuel goes into the float bowl, and the pressure drops some small amount, until the fuel pump restores the pressure.

    The fuel pump has plenty of excess capacity to both supply the engine and return some fuel to the tank. As mentioned, this increases flow through the fuel lines and lowers the temperature of the fuel in the lines. IMO it's worthwhile to put the return line back so that vapor lock is less likely. The orifice is also said to release any vapor from the lines by venting it, so the anti-vapor-lock function may be more than just keeping the fuel cooler.

    If you put the return line back, use some fuel line that won't deteriorate, ie not rubber fuel hose. I'd use steel since it's cheap and sturdy, but possible you could use modern nylon line or such. Long runs of rubber fuel hose are a definite fire hazard.

    Also to the OP - I suggest you start a new thread with the speedometer question. It's your thread, but if anyone else changed the topic like that, it would be a hijack.
     
  8. Oct 18, 2019 at 2:23 PM
    Dale Boyd

    Dale Boyd New Member

    Pekin, Indiana
    Joined:
    Jan 8, 2018
    Messages:
    31
    Thanks for the feedback on the return fuel line.
    I will move the speedometer question to a new post.
     

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