98 Gt Cutting Out At 4000rpm

Discussion in 'SN95 4.6L Mustang Tech' started by Cody5791, May 28, 2014.


  1. Cody5791

    Cody5791 New Member

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    My 1998 ford mustang gt 4.6 starts cutting out at 4000 rpm if accelerating hard at all. However it doesn't cut out at all if I'm just slowly accelerating only when getting on it hard. Doesn't miss or throw any codes besides downstream oxygen sensor codes since the oxygen sensors were removed when x pip was installed. Any ideas?
     
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  2. wmburns

    SN Certified Technician

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    Start out by checking for moisture in the spark plug wells.

    How old are the spark plug wires? Have all factory stand offs and looms been used? Followed factory routing?

    Try this old racer's trick. Run the motor in a totally dark location. Look for the blue hue of escaping spark (arcing). Inspect the spark plug wires for white spots which is evidence of escaping spark. Replace if any is found.
     
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  3. Cody5791

    Cody5791 New Member

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    Ok ill try that. I did put a fuel pressure tester on it today and it was holding 30 at idle. So I let it sit for around 10 minutes and it didn't change. Then I revved it up till it started cutting out and it may have made it to 31 psi. Then i checked it while driving the car and it didn't go up and when I parked the car and let it idle it was actually about 28-29 psi.
     
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  4. wmburns

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    Is the fuel pressure regulator intake vacuum reference line connected and leak free?

    For a test, disconnect and plug the fuel pressure regulator intake vacuum reference line? Does the fuel pressure go to 40 PSI?

    Are you familiar with how the fuel pressure should behave when measured with an external gauge? When measured with an external gauge, the pressure should be 40 PSI as indexed by the intake manifold. As the intake manifold is under a vacuum at idle, that is why the fuel pressure is around 30-32 PSI at idle.

    HOWEVER, as the throttle is opened, the intake vacuum goes down (pressure goes up). Therefore, the gauge fuel pressure should go UP by the same amount. Since your fuel pressure does not change as the throttle is opened, that tells me either:
    • The fuel pump is not able to keep up with the motor's requirements. NOTE, the 98 GT uses a two speed pump. IF the pump is stuck in low speed mode, this could explain your symptom.
    • the fuel pressure regulator is not connected to the correct vacuum source. IE the vacuum is not changing with throttle position/engine load as it should.
    • The fuel pressure regulator is bad. IMO this is a long shot in your case.
    It would help to know the voltage at the trunk mounted IFS switch. We need to know the voltage before start, as the motor is started, and as the motor RPM's are increased. At some point, you should see the voltage increase when the PCM switches the fuel pump from low speed to high speed mode.
     
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