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Fuel Pressure- Low key off, okay at idle

Discussion in 'Under the Hood' started by Dannyboy, August 3, 2008.

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  1. Dannyboy

    Dannyboy Other Make & Models Mod Elite Explorer Moderator Emeritus

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    While testing my fuel pressure I have ran into an issue that the "search" feature is not helping me find the answer.

    1991 Explorer 4.0

    *just replaced fuel pressure relay
    *fuel pump is a little noisy
    *lacking power at wide open throttle


    Key on, Engine off---Fuel jumps near 30 then falls below 10

    Key on, Engine running--Fuel pressure sits around 30

    Key on, Engine at high RPM--Fuel pressure still sitting around 30


    Any clue why I have no pressure at the rail when I run the test? This is the same regardless of how many times I cycle the key.
     
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  3. Tbars4

    Tbars4 Moderator Emeritus Elite In Memoriam

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    ...Did the relay resolve the code??...I take it that it did not...

    ...I'm thinking that your fuel pump may be ready to go...:scratch:...To test mine I would just find a steep hill and try to go up that...Usually this would result in it acting like you are running out of gas...

    ...You might check for fuel in the vacuum line at the fpr, as I take it you have found no external leaks...:dunno:

    ...Good luck with this and a bump for one of the experts to chime in for you..;)
     
  4. Tbars4

    Tbars4 Moderator Emeritus Elite In Memoriam

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  5. Dannyboy

    Dannyboy Other Make & Models Mod Elite Explorer Moderator Emeritus

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    Thanks Ted, that's what I'm up in the air about, I can't find any evidence if it's my fuel pump or fuel regulator. I should probably just change both.

    Just for kicks I recorded my test on video.

    This took a little longer to start than the other times. This is one advantage of a 4 pin removable hood.



    So, I tried this site called Google out with a search and came up with this


    Residual Fuel Pressure Test
    When the pump is turned off or stops running, the system should hold residual pressure for several minutes (look up the specs to see how much pressure drop is allowed over a given period of time). If pressure drops quickly, the vehicle may have a leaky fuel line, a leaky fuel pump check valve, a leaky fuel pressure regulator or one or more leaky fuel injectors. Low residual fuel pressure can cause hard starting and vapor lock during hot weather.


    So, I don't have any leaking gas so I guess I can rule out
    1) leaky fuel line

    So now I need to search for
    2)leaky fuel pump check valve
    3)leaky FPR (fuel pressure regulator)
    4)leaky fuel injectors
     
    Last edited: August 3, 2008
  6. 90ranajo

    90ranajo The Little TJ that could

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    I believe the VIP DLC is the OBD-I connector. There should be one under the hood somewhere (I think around the core support area).

    Fuel Pressure Test Procedures:
    [​IMG]
    [​IMG]
    [​IMG]

    Here's a exploded view of the Fuel Pressure Regulator (FPR)
    [​IMG]
     
  7. JTX

    JTX Well-Known Member

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    The fuel pump should be staying on longer. Mine stays on for at least five seconds. You can also hear my pump slow down as the pressure builds. If i do it twice in a row the pump doesnt even come on because theres still pressure. Hope that makes sense. lol
     
  8. Tbars4

    Tbars4 Moderator Emeritus Elite In Memoriam

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    ...The obd-1 connector is found in the harness by the heater fan...;)

    ...I'm going to have to bookmark this thread..:thumbsup:
     
  9. 90ranajo

    90ranajo The Little TJ that could

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    You also may want to check your EEC Power Relay. As that little bugger sends the Batt+ to the Fuel Pump Relay, Ignition Control Module, A/C system, MAF sensor, A/C WOT Cut-out relay, Canister Purge Solenoid, PCM pins 37 & 57, fuel injectors, CPS, EGR Vacuum Regulator solenoid, ISC solenoid, & the TCC & 3-4 Shift Solenoids.
     
  10. EMG7895

    EMG7895 Well-Known Member

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    If you do change the fuel pump and need a new lock ring pm me I have an extra from when I changed mine and ordered 2 by accident. To rule out the fuel pump unplug the connecter behind the tank on the frame and jump 12v to the pump and then try to run it, that would rule out the pump and point to the circuit.
     
  11. 90ranajo

    90ranajo The Little TJ that could

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    I think that's correct for the Rangers but on Explorers it looks to be on the driver side fender behind the battery:[​IMG]
    We're looking at C198 on the right hand side.
     
  12. Tbars4

    Tbars4 Moderator Emeritus Elite In Memoriam

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    ...I think that drawing might be from a 93-94...The battery and power distribution box are on the passenger side on 91-92's..:dunno:
     
  13. Rick

    Rick Pumpkin Pilot Staff Member Admin Elite Explorer

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    Dan, is your fuel pump still really loud?
     
  14. 90ranajo

    90ranajo The Little TJ that could

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    That's what comes up for a '91 from the ARRC (Automotive Repair Reference Canter) site.
     
  15. Dannyboy

    Dannyboy Other Make & Models Mod Elite Explorer Moderator Emeritus

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    Umm, kinda, but not as bad as it was the other day. I know this is totally unrelevant, but maybe more relavant than I'm thinking, but I just got back from autozone with another new battery. I exchanged an optima that tested bad. I've been running the truck off jumper cables all day and after that post from EMG7895 and 90Ranajo I decided I needed to eliminate that possibility.

    And on a 91 the hookup is on the passenger side right behind the power distrubution fuse block and relay block.

    Here it is hooked up to the ridicuosly cool XP240 scanner that I just found in my parts storage a few hours ago.

    Back to testing. I'm sure learning a lot (again).
     

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  16. Tbars4

    Tbars4 Moderator Emeritus Elite In Memoriam

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    ...While you are testing, did you check the info in post #3???
     
  17. Dannyboy

    Dannyboy Other Make & Models Mod Elite Explorer Moderator Emeritus

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    Hooray for SHOW. His diagram and test determined I need a new fuel pump

    A1-yes, go to A10
    A10-No, go to A11
    A11-Yes, go to A12
    A12-No-Replace Fuel pump, go to AI

    So I seriously spent most of the day putzing around with this, I got a new battery and a new battery mount in the meantime and went through and checked and tightened all the bolts of my suspension, but I spent way longer on this than I would have expected. Thats alright though, I would have been mad to have spent $80 on a regulator and not cure my problem or go through all of the trouble of putting in a pump and then having the same problem.

    I think I'll drop my tank this time since I need to have some reshaping done on my skid plate and fuel tank assembly (it is close to my rear driveline.

    Also, I bought MORE fuel line tools. Now I have 2 kinds and I STILL can't get the return lines off. Any suggestions? I screwed up and snapped the return hose right at the 90 degree bend above the fuel pressure regulator so even though that part is good, the hose is not. What a PITA. I got these guys cause the other one's wouldn't fit. I'm pushing and pulling and having no luck.
    [​IMG]

    also tried using these but there isn't enough force
    http://photos.imageevent.com/eelpusdet/miscpics/fuelfilter8.jpg

    and I've tried to use these before and snapped them in half
    [​IMG]


    Any suggestions? I am going to soak the springs with PB Blaster
     
  18. 90ranajo

    90ranajo The Little TJ that could

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    I'm here for you, Danny!

    Those lines are just a pain in arse. Some will come off easy, others will just fight you for hours.
     
  19. Evil Eye

    Evil Eye Active Member

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    Good deal, Dan! I feel you on fighting those connectors. I'm doing the AC right now and having my own share of challenges with those...
     
  20. JTX

    JTX Well-Known Member

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    I kept breaking the plastic ones and finally went with the metal ones and got it. Get the metal one and beat it in with a hammer. Thats what i did. PITA no matter what though.
     
  21. Dannyboy

    Dannyboy Other Make & Models Mod Elite Explorer Moderator Emeritus

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    I haven't looked too hard yet BUT, I'm guessing this is a dealer only part from what I'm seeing.

    Dealer wants $403.27 for a hose that is 2 feet long.

    I broke it right here at the 90 elbow running the fuel pressure tests.

    I guess I'm going to the junkyard to part out a friggen fuel hose. I can't even get the first one off, now I'm trying to get a second one off.

    Can I pull the FPR and hoses from a 93-94?

    (disregard the green circle, I stole this pic from my gauge thread)
     

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