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my 2000 gt idles high at times, i have changed the iac, cleaned the map, cleaned the throttle body, just bought a puel pressure tester and found with just key on its about 8 psi pressure when started it fluxated between 30 and 34 with high idle above 1 gran, after a few min idle went to about 900 and pressure flux between 30 and 34, sometimes when coming to a stop have to hold the brake hard because it idles to high. i was wondering if the fuel pressure is correct or have a bad pump?
 

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The fuel pressure should be 40 PSI intake referenced (delta). This is important! At idle the intake is under a vacuum. So it it NORMAL for the fuel pressure to be lower.

On a return fuel system, testing fuel pressure with the motor off is not very useful.

To correctly test the fuel pressure with an external gauge, disconnect and plug the fuel pressure intake vacuum reference line. The fuel pressure should jump to 38-40 PSI.

Note, never drive the car with the intake reference line disconected.

For more information on how to trouble shoot idle problems, check out:

http://www.allfordmustangs.com/forums/4-6l-tech/49661-your-mustang-stumbling-dieing-idle-idling-rough-14.html#post2260649
 

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Discussion Starter #3
The fuel pressure should be 40 PSI intake referenced (delta). This is important! At idle the intake is under a vacuum. So it it NORMAL for the fuel pressure to be lower.

On a return fuel system, testing fuel pressure with the motor off is not very useful.

To correctly test the fuel pressure with an external gauge, disconnect and plug the fuel pressure intake vacuum reference line. The fuel pressure should jump to 38-40 PSI.

Note, never drive the car with the intake reference line disconected.

For more information on how to trouble shoot idle problems, check out:

http://www.allfordmustangs.com/forums/4-6l-tech/49661-your-mustang-stumbling-dieing-idle-idling-rough-14.html#post2260649
is the intake reference line the vacuum line that connected to the fuel rail and has the fuel pressure sensor in it?
 

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is the intake reference line the vacuum line that connected to the fuel rail and has the fuel pressure sensor in it?
Yes. The fuel pressure sensor has an electrical connector and a vacuum line on the fuel rail.
 
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