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Fuel Pressure low?

Discussion in 'Under the Hood' started by #1Aggie, September 25, 2002.

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  1. #1Aggie

    #1Aggie New Member

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    My '93 explorer has been trying to stall lately. While I have whole list of items to do this weekend, tonight I checked the pressure on the fuel rail.

    1. With key on, but engine not running = 40 psi
    2. With engine idling = 30 psi

    All data.com specs out 35-45 for #1 and 30-40 for #2.

    While both of my numbers are within the range, the running one is on the bottom end.

    I am planning on changing my fuel filter this weekend to see if the pressure comes up any. Or do I have a fuel regulator failing?

    Has anyone else ever checked their fuel pressure? If so, what numbers did you get?
     
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  3. Opera House

    Opera House Well-Known Member

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    Fuel Pressure Regulator

    I had problems with my 92 stalling and it was the fuel pressure regulator. After I tried everything else, ordered the fuel pressure regulator. After installing the new one, I cut open the old one to verify the cause of the problem. A flat hardened metal plate covered the valve orifice. The pressure spring on the plate was slightly off center. This caused a 30 degree bevel to be worn on the orifice surface on one side only. At idle, increased vacuum caused the plate to lift up more and occasionally slide to the side on this bevel. This sometimes held the valve open causing very low fuel pressure. When the engine would violently shake just before stalling, the plate would be reset to a normal position and everything would be fine for a while. When the engine ran fine the pressures were normal. Your pressures look fine.

    I first thought it was an electrical problem. Pulled the fuel pump fuse and put connectors on some lamp cord. Stuck the connectors in the fuse socket and brought the wire into the cab. Monitored pump current on a DVM 10A range. Normal pump current is about 5A. Just before the engine would stall, you would see the current slowly drop to 2.5 A. This is a high bypass fuel system and current roughly mimicks pressure. Expect to see only about 0.6A change from high to low.

    Easy replacement and about $50 aftermarket. Disconnect battery after replacement. My computer didn't relearn for 200 miles till I removed power.
     
  4. #1Aggie

    #1Aggie New Member

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    Re: Fuel Pressure Regulator



    This sounds exactly like what mine is doing. First it is intermintent. Secon, it only happens when I am starting from a dead or slow roll and my car bucks like a bronco and then recovers.

    When I turn my key on, but without the engine runnning, the pressure comes up to 40 psi and then falls off.

    Thanks, I will give this a try this weekend.
     
  5. Opera House

    Opera House Well-Known Member

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    Not exactly

    Fuel pressure drops with manifold vacuum. This is the equivelent of the accelerator pump. Step on the pedel and more gas, because of higher fuel pressure, is injected. Pull off the vacuum line to the regulator at idle and the pressure will shoot up. I don't trust a lot of pressure gauges except my electronic one that is 0.5% accurate. Some gauges can be off 7#. Normally max pressure is about 39# and with vacuum it will drop 6-7#.

    I would expect the problem to happen just after coming to a stop and not accelerating. This is when the manifold vacuum is the highest. It would also stumble at the same high speed right turn every day when I drove home from work. Suppose that was due to centrifical force and taking my foot off the gas. Humans are so adaptable. I learned to deal with it for about six months. It did die crossing the road one time when I was pulling a trailer and it also died on a highway and I couldn't get it to restart for a half hour. That was the last straw. It also seemed to have less power going up hills near the end ans was more prone to pinging. One good thing about the problem was it greatly improved gas mileage. Guess I babied it more to prevent pinging. Can you hear your fuel pump and if you let it idle in the driveway does the tone change?
     
    Last edited: September 26, 2002
  6. #1Aggie

    #1Aggie New Member

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    Re: Not exactly

    Yes. It has the normal high pitch whine and it is steady.
     
  7. Opera House

    Opera House Well-Known Member

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    The weekend is almost here

    If you got a voltmeter, I'd measure the pump current. It may be delivering the pressure but not the flow necessary. This is a high bypass system and if the current is low then it could be that the pump gears are worn or there could be some electrical resistance going to the pump. A lot of pumps will run a car with as little as 6 volts. I once ran a pump in a 5 gal plastic bucket in the back seat to get the car home. To prevent it from pumping most of the gas into the cars tank in no time, I connected a brake lamp in series with the fuel pump power wire. Just couldn't do heavy acceleration. The things we do to save a tow charge. Have you tried driving around with that pressure gauge? We may be barking up the wrong tree looking at fuel.
     






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