An Odd Problem?

Discussion in 'Classic Mustang Specific Tech' started by horseballz, Jun 22, 2013.

    1. Hey Folks,
    On hot days (100-115 degrees) when I stop at traffic light, idle is initially fine but gradually, over 20-40 seconds deteriorates, drops and stumbles a bit and then after almost wanting to stall, it picks back up to normal, for about 20-40 seconds and if I sit stopped, idling (especially with AC on) it repeats this cycle again and again. Before I explain my observations, I will detail my fuel delivery configuration:

    A-Stock type mechanical fuel pump that gives a steady 6 3/4-7 1/4 PSI.
    B-Steel line from pump to a clear bodied fuel filter.
    1-steel line is connected to the filter with a short length of rubber hose. The nipple of the filter basically meets/touches the steel line and the hose simply seals the connection.
    C-Out of filter to a short steel line going into a QFT fuel pressure regulator, set to 5 3/4-6 PSI. Again, a short piece of rubber to seal the connection.
    D-Out of fuel pressure regulator into a QFT-580VS carburetor (hard plumbed) with sight glasses on the float bowls.

    So here is my test/observation sequence:

    1-idling fine with AC on, engine well warmed on a hot day.
    2-Fuel bowl level starts to visibly drop slightly.
    3-Clear filter starts to get a few bubbles in ita-Idle begins to deteriorate.
    4-Fuel bowl level continues to drop slightly.
    a-Filter fills with bubbles.
    b-Idle continues to deteriorate.
    5-Fuel bowl slowly fills back up to normal level
    a-Filter bubbles are gone
    b-Idles improves back to normal
    6-After 20-40 seconds this cycle begins and repeats, again and again, etc..........

    Now I'm making a semi-educated guess that the problem is "boiling" fuel. My real question is, what process "exactly" is causing it and how can I cure it? Obviously heat is at the root, but is it:

    A-Fuel under pressure from the float needle/seat closed having a higher boiling temp?
    1-Float opens, dropping the pressure and thus boiling temp.
    2-Bowl begins to refill slowly because of the boiling bubbles.
    3-Float closes, thus increasing pressure/boiling temp.

    Or do ya think:

    A-Fuel in line gets to sit long enough to boil and then when the float opens.
    B-The cooler fuel from the tank cures the issue?

    Its sort of "which came first, the chicken or the egg" kinda question. View attachment 127716 What the actual cause is will have a significant impact on what the solution is.

    Any and all thoughts comments and/or suggestions will be sincerely welcomed.
  1. it does sound like a fuel boil issue. it can be caused by many things, but the most obvious are things like a lack of an insulator between the carb and the intake, the fuel line is too close to the outlet of the cooling system, etc. look to areas like these first.
  2. Been there, done that. 1" phenolic spacer under the carb. I will try rerouting the fuel line to gain a bit more clearance from the thermostat housing/upper hose, but no matter what I do, the air coming through the radiator/AC condenser is BLAZING HOT! I may try the old heat sink trick of wrapping the fuel line in aluminum foil.:shrug: Also wondering if increasing the fuel pressure before the regulator might eliminate the boiling in the filter/line and possibly get liquid fuel to the carb inlets instead of boiling/bubbling fuel? Remember that with ambient temps in the 110 degree range, the temp at 2 ft above the hot black pavement (approx. center height of radiator/engine) is likely 125-130 or more!:eek: Extreme conditions necessitate imaginative solutions.
    Thanx For The Thoughts,
  3. Supposed to be 112 next weekend. Time for the torture test.

    Sent from my EVO using Tapatalk 2
  4. Hey jetmech807,
    What part of the oven are you located in? PM me and maybe we can meet up for a cold one and some chat!
  5. possibly installing a high pressure electric fuel pump with a return line back to the tank that keeps the fuel quickly flowing and a fuel pressure regulator at the carb to keep it at right pressure. probably a $200 job, but I think it would work if boiling is your problem. maybe even adjust your float to hold slightly more fuel in the bowl. Also I would recommend getting the biggest radiator you can find. and a trans cooler. those 100 plus days are murder on engines and trannys.
  6. i think i would try an electric pump as well,mounted at the rear of the car were heat should be quite a bit less. just imagine how hot that manual pump must get right next to exhaust manifold / headers. especialy if it is in the 100's or higher :drool:
  7. instead of wrapping the fuel line in aluminum foil, a better idea would be to make a heat shield that blocks direct airflow over the fuel line. i do like the idea of an electric pump feeding the carb though and eliminating the mechanical pump. that way you get a constant flow of fuel to the carb rather than a pulsating flow.
    1. I have a 1" phenolic spacer under the carb and until I lifted the rear of the hood 3/4"-1" for heat dissipation, I would still have hot starting issues, especially when parked for 15-30 minutes. I had another thread going concerning this issue and will eventually be putting in some hood louvers to help evacuate the heat. The raised rear of the hood cures the problem but looks FUGLY!
      As far as electric fuel pumps:

      A-I'm not sure that I know how to plumb one with a regulator and return line.
      B-It has been my understanding that a "non return" style electric pump that is "always running" has a limited life expectancy.
      C-It seems that the best and easiest electric set up would be a pump near the tank, set to cycle on and off at a set pressure (say, 8-10 psi) with a regulator at the carb set to the recommended 6-6 1/2 psi. Does anyone here have a suggested pump like this, that will be durable and not break the bank? As stated above, I already have a regulator at the carb.
  8. plumbing a return line is easy. you just use one outlet port in the regulator to supply fuel to the carb, and the other outlet port send fuel back to the tank, or you can use a T fitting set up just before the pump inlet and run the return line there so you dont have to modify the tank.
  9. This doesn't sound like an odd problem, it sounds like a pain in the butt.

    My Boss has problems with heat so I am changing the hood to the 302R spec hood this year when I have the cash. The air flow into the engine bay is restricted and the air flow out of the engine bay is restricted so heat tends to build up and cause the cam sensors to fail. One thing people forget is that all that cool air going into the engine bay to cool the radiator off has to go somewhere. If it can't get out of the engine bay efficiently, it will heat everything up even more.

    Do you have the upgraded turn signal hood or the standard hood on your 68 ? If you put the turn signal hood on you can vent the engine bay through the turn signal holes. Might not help so much at idle however....

    I agree with rbohm, you should insulate the fuel line where possible. Also, instead of having the fuel line run up the front of the block by the timing cover/water neck maybe you can bring it up by the valve cover and run it in an area with less potential heat soak.