98 Gt Cutting Out At 4000rpm

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.

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.