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Standard Fuel Pressure

Discussion in 'Under the Hood' started by raiper, December 17, 2004.

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    1. raiper

      raiper Active Member

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      I was just wondering what the standard fuel pressure should be at idle and should it fluctuate at all or stay still. Also does it stay at a consant pressure at times including when having the engine on a load?
      My problem is it Flucutates at Idle from about 37-43 psi and then under load it drops to around 20-25 psi and that to me doesnt sound right
       
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    3. Jefe

      Jefe Well-Known Member

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      91-98 4.0 V6 should be in the 40-42 lbs range all the time.
       
    4. MrShorty

      MrShorty Explorer Addict

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      It should hold steady at ~40 psig with the vacuum line to the FPR disconnected. With vacuum applied to the FPR, the fuel pressure will drop some, but it shouldn't drop below 30 psig. For fun, I once was playing with my '92, and the engine would stumble and stall if I let the fuel pressure get below ~25 psig. Your test suggests that there is something wrong upstream of the fuel rail (clogged fuel filter, kink in a hose, weak pump)
       
    5. raiper

      raiper Active Member

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      how bout air being in the line
       
    6. CDW6212R

      CDW6212R Hauls the mail. Elite Explorer

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      I had a stumble off idle, which was from a vacuum leak. The rubber line at the fuel pressure regulator had swollen from fuel(occasional backfire cough on startup). My pressure was too high at idle.
      This isn't your problem, but maybe someone else may have this symptom. Good luck,
      DW
       
    7. Opera House

      Opera House Well-Known Member

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      The fuel system in the first and second gen explorers is high bypass. That means more than 80% of the fuel goes back to the tank. Given that, you can have severe pump problems and the engine can still run. The fuel pressure is controlled by manifold vacuum. Pressure is reduced by EXACTLY the manifold vacuum. In most cases that can never be more than 8# so 32-34# is the lowest pressure you will see at idle. If pressures drop below that it is likely you need a new pump. You could monitor pump current as you drive around and look for big drops in current. Shouldn't go much below 4A. I've run pumps on less than 6V and the engine still ran. That would be almost 3 ohms in resistance to the pump. I'd check connectors , fuse and the pump relay.
       

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