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Fuel Pressure Regulator operating correctly?

Discussion in 'Stock 1991 - 1994 Explorers' started by iteotwawki, June 23, 2011.

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    1. iteotwawki

      iteotwawki Active Member

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      I'm hoping to get some input regarding the function of the vacuum line on the back of the FPR.

      I am having a problem with performance in that my truck will intermitantly bog down (happens more often when it is hot) and it will also backfire "cough" when I mash the accelerator at a stop. Anyway, I was checking for vacuum leaks today and I pulled the vacuum line to the FPR and caped the outlet on the Intake and didn't notice any difference in the way the truck ran. Is this normal?

      Going to check the fuel pressure tomorrow and report.
       
      Last edited: June 23, 2011
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    3. Masterd

      Masterd Active Member

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      Im gonna subscribe to this one. Way back when I found the line sitting off of my regulator and it never made a difference in how it ran, which is odd.
       
    4. MrShorty

      MrShorty Explorer Addict

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      That is normal.

      When it is operating as designed, the FPR moderates the fuel pressure based on "engine load" as measured by engine vacuum. When the vacuum is high (indicated a light load), the fuel pressure is low (low 30's psi). When vacuum is low (indicating a high engine load), the fuel pressure increases (up to ~40 psi). This pressure change is not enough to change how the engine runs, as the PCM adjusts injector pulse width to get the desired A/F ratio.

      When the vacuum line is disconnected and capped, the fuel pressure runs at a constant value (~40 psi) and doesn't (shouldn't) change. The PCM sends a slightly different pulse width to the injectors to maintain the desired A/F ratio. As long as the pressure stays within a reasonable "working" range, the PCM can adjust the pulse width to get the desired A/F ratio.
       
    5. iteotwawki

      iteotwawki Active Member

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      Thanks for the detailed responce.
       
    6. Roadrunner777

      Roadrunner777 Well-Known Member

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      I'm going to suggest a refinement of this idea of the vacuum line on the FPR. I read this a few months ago and I don't have the source anymore, sorry for that, but here is the idea:

      The ideal situation is a perfect spray pattern for the correct duration. Assuming a new/clean injector, the only factor in spray pattern is fuel pressure. We have all used a nozzle on the end of the hose and found the 'sweet spot' where is provides just the desired spray. Fuel injectors have the same sweet spot.

      Here's the hitch... put your science hats on. The fuel injectors spray into the manifold. The manifold can be almost at atmospheric pressure at WOT, or at a vacuum at idle. The fuel spray pattern is therefore, a combination of fuel pressure and manifold vacuum.

      The point of the vacuum line to the FPR is to compensate for the change in manifold vacuum, so the pressure differential (Fuel to manifold vacuum) is the same in all conditions, and produces the same perfect pattern regardless of manifold vacuum.

      As Mr. Shorty says, the computer can compensate for the lack of this function by changing the injector duration. So, the engine runs and performs. But, if you do not run this line, the spray pattern will be less than optimum, and you may not be getting all the mileage and power that you could be.

      And I am very definitely splitting hairs. I'm not sure what manifold vacuum is in psi units, it would be something like -5 PSI at idle, maybe? And I know I have seen fuel pressures posted here that are all over the place, so the effect of this vacuum line is probably negligible. But, if you are a perfectionist looking for that last mpg or HP, you will need this line working properly.
       
    7. iteotwawki

      iteotwawki Active Member

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      The resolution

      I stoped by my friends shop and tested the fuel pressure and according to the gauge I have 40 lbs. pressure consistantly.

      However, it turns out it was not a faulty fuel pressure regulator or anything fuel related. The number 4 spark plug had two cracks in the porcelain under the plug wire. Also the number 5 plug was a little caked with carbon from when I had oil in the cylinder due to loose lower intake bolts.

      Six new Motorcraft spark plugs and my explorer purs like a kitty. Awesome! Thanks for the help everyone. Much appreciated :D
       
      Last edited: June 25, 2011

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