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No_Lemon's avatar
No_Lemon
Explorer
Sep 11, 2015

1994 p30 Winnebago warrior fuel pressure issue

Just got back from a short trip due to loss of power issue.

1994 Winnebago 454 fuel injected motor.

We had traveled 80+ miles up a couple pretty good inclines with no problems. Had plenty of power and running smooth. We came into a small town where the elivation was about 3500 feet. The engine started acting like it was running out of fuel. If you gave it a little to much throttle it sounded like it was just sucking air and would slow down. If I pumped the gas pedal a couple times it would take off for a few seconds then start bogging again.



I had 3/4 tank of fuel but decided to fill it up just incase. Same thing.

I changed the fuel filter and same thing so I figured it was the fuel pump. I installed a fuel pressure gauge and it showed around 5psi. I installed a new fuel pump and same low pressure so I figured it might be the fuel pressure regulator. I got a rebuild kit and put it in and same problem.

If I blocked off the return line pressure would jump to 50+ psi.

I ended up putting some vice grips on the return line and adjusted them so I had about 20 psi. Took it for a test ride and it ran pretty good so I headed back home and made it.

After I got home I unplugged the fuel pump harness at the tank and connected 12volts and same low fuel pressure.


Any idea why I have such low fuel pressure?
Class C
  • More_To_See's avatar
    More_To_See
    Explorer
    Sep 14, 2015
    That almost seems to have been a glitch in the computer (PCM). Let's hope it's a permanent fix.

    I'm fighting a problem with my smog system to renew the license plates. Seems to be in the EGR area but of course sometimes stuff is inter related. I pulled the valve out and it buzzed out ok best as I can tell. It's the expensive linear type with a motor and a pintel connected to a rheostat and I don't have it as a spare part. Most everything else under there I do carry. I've even got a spare alternator.

    My principle is that if you carry it as a spare it ain't never gonna break.

    I sure hope my problem doesn't provide to be as obscure as yours.

    Good travels.
  • No_Lemon's avatar
    No_Lemon
    Explorer
    Sep 13, 2015
    I unhooked the temp sender that goes to the computer and when I started it I got a service engine soon light. I took it for a five mile ride and it ran perfect. I then turned it off and hooked up the sensor and when I started it i had no service engine light. I went for two five mile rides and it ran perfect both times?
  • More_To_See's avatar
    More_To_See
    Explorer
    Sep 13, 2015
    Possibly things are going South when the engine goes from open loop to closed loop.

    With a cold engine (as in not up to operating temp) not all your engine sensors and controls are being used. They are set to a cold / default condition.

    So the question is can we narrow it down to which ones.
  • No_Lemon's avatar
    No_Lemon
    Explorer
    Sep 13, 2015
    Update.

    Still doing the same thing. I have determined that after start up and driving about 1 mile it starts to act up every time.

    After determine the fuel pressure should be 9-13 psi I changed to a low pressure gauge and found that my 0-100 psi gauge was playing tricks on me. What I thought was 5-6 psi was really 12-13 psi on the lower pressure gauge so I think replacing the filter, regulator and fuel pump were not needed at all?

    Moving on to what else it could be I removed the factory cat and still the same, I unpluged the O2 sensor and same thing.

    Things left.

    Computer, tps, temp sender, O2 sensor, iac, bap sensor or map sensor, air inlet sensor?

    Keep in mind this thing runs perfect for about 1 mile then starts acting up.
  • enblethen's avatar
    enblethen
    Nomad
    Sep 12, 2015
    1994 7.4 throttle body should be 9-13.
    26-32 is for later engines.(vortec)
  • More_To_See's avatar
    More_To_See
    Explorer
    Sep 12, 2015
    7.4 liter is 26 to 32 psi

    Edit: My info is out of a Gm 1994 service manual for a P3 & P/G Chassis. This engine is not Vortec. The OP's engine is likely a 1993 version.

    EDIT 2: after checking some repair kits online for a 93 engine a couple of them say 10 psi. So perhaps I stand corrected and need to mark up my Manuel.
  • No_Lemon's avatar
    No_Lemon
    Explorer
    Sep 12, 2015
    I just noticed the new regulator had a threaded insert it the bottom of the regulator. I threaded a bolt into it and have raised the fuel pressure to 15 to 16 psi. I have not drove it yet as I am trying to find the correct pressure for it. Does anyone know the spec for fuel pressure?
  • More_To_See's avatar
    More_To_See
    Explorer
    Sep 12, 2015
    Previously you "fixed" the problem by manually restricting the fuel return line to the tank thus increasing pressure to the injectors.

    Does that still work?

    If so, it seems to me that would strongly indicate a pressure issue and not much else.

    The throttle position sensor controls the amount of fuel to the injectors with pulse width modulation but I don't know that that is your problem. The sensor provide 0 to 5 volts back to the computer letting it know how wide open the throttle is.

    Keep in mind that your fuel pump in the tank receives voltage through the oil pressure sender when the engine is running so if there was a problem there the pump might only run occasionally.

    I might start wondering about whether the engine computer is somehow monitoring that area and has a problem. Maybe remove battery cable for an hour or so and let the computer clear itself and then start all over. But that's a wild a** guess.
  • enblethen's avatar
    enblethen
    Nomad
    Sep 12, 2015
    Test the throttle position sensor!
    Check the crankshaft position sensor
    Check the camshaft position sensor
    Even though the engine check light is not on, have you checked for codes? They do not always turn on check light.

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