Fuel Pressure??

blue46gt · Nov 27, 2004

Does anyone know what my fuel pressure guage should be reading. I thought it was supposed to read 35psi at idle but it's only reading 30psi. I cleaned my MAF wires and I'm still having cold starting problems. I'm starting to think it's some kind of fuel problem? This is getting really frustrating.

GT2000JAY · Nov 27, 2004

30 PSI is normal at idle. Check it under load - does it climb and remain stable under throttle? If you go WOT and it climbs and drops or fluctuates quite a bit then you may have a fuel problem.

Jay

blue46gt · Nov 27, 2004

Thanks Jay. I'll take it out tomorrow and check that out.

YtnGT · Nov 28, 2004

You might have to think about this a little before it is clear. Fuel pressure is always maintained at 39psi across the injector. That means the difference between one side of the injector to the other is 39psi. Remember at idle you have quite a bit of vacuum in the intake manifold and top side of the heads. That vacuum is trying to suck the fuel right out of the injectors. At WOT the intake vacuum should be close to zero and you should read approx 39psi on your gauge.

Moral of the story,for the stock fuel pressure sensor, fuel pressure is referenced to manifold absolute pressure (vacuum). Your gauge is referencing fuel pressure to atmosphere.

Maybe a mechanical engineer will chime in and explain this better than I did (I am just a dumb electrical guy).

CanadaStang · Nov 28, 2004

29-32 is normal at idle with the vacuum line on, 38-40 with the vacuum line off.

blue46gt · Nov 28, 2004

What do you mean when you say with the vacuum line off? Is this a mod of some kind, what are it's advantages? Thanks!

2002BLGT · Nov 28, 2004

99 and newer cars dont have a vacuum referenced FPR there is a sensor on the fuel rail that senses vacuum for load and adjust fuel pump pressure through the ECM .....30 at idle is fine ........

CanadaStang · Nov 28, 2004

2002BLGT said:
99 and newer cars dont have a vacuum referenced FPR there is a sensor on the fuel rail that senses vacuum for load and adjust fuel pump pressure through the ECM .....30 at idle is fine ........

Whoops, my bad didn't notice it was a 2000

YtnGT · Nov 28, 2004

99+ have a fuel pressure sensor referenced to manifold absolute pressure (vacuum). That way ford only has to use one sensor and they are measuring the differential pressure across the injector. It is a really simple but ingenious way of measuring differential pressure.

2002BLGT · Nov 28, 2004

YtnGT said:
99+ have a fuel pressure sensor referenced to manifold absolute pressure (vacuum). That way ford only has to use one sensor and they are measuring the differential pressure across the injector. It is a really simple but ingenious way of measuring differential pressure.

correct... fuel pressure (keyword SENSOR) thats how I was trying explain it , the regulator is actually the ECM by changing the voltage to the fuel pump ....

YtnGT · Nov 28, 2004

2002BLGT said:
correct... fuel pressure (keyword SENSOR) thats how I was trying explain it , the regulator is actually the ECM by changing the voltage to the fuel pump ....

Exactly...

I think everyone, with a 99+ 4.6, needs to know that the computer is actually the fuel pressure regulator.

fuel pressure sensor (controller input) to Computer (PID control loop) computer output (PWM variable voltage, aka controller output) to fuel pumps and affects fuel pressure

GT2000JAY · Nov 28, 2004

YtnGT said:
Exactly...

I think everyone, with a 99+ 4.6, needs to know that the computer is actually the fuel pressure regulator.

fuel pressure sensor (controller input) to Computer (PID control loop) computer output (PWM variable voltage, aka controller output) to fuel pumps and affects fuel pressure

Off topic, but which is also the reason why it is bad idea to use a pump not designed for variable voltage (walbro, etc.) in a returnless car. Stock pump, focus pump, aviator pump (which is what I use) is the way to go, given that the computer can modulate the voltage to the pump and generate enough amperage to drive the larger pumps.

Jay

2002BLGT · Nov 28, 2004

GT2000JAY said:
Off topic, but which is also the reason why it is bad idea to use a pump not designed for variable voltage (walbro, etc.) in a returnless car. Stock pump, focus pump, aviator pump (which is what I use) is the way to go, given that the computer can modulate the voltage to the pump and generate enough amperage to drive the larger pumps.

Jay

aviator pump is the best because the current draw on it is less than the GT stock pump , the focus pump has about the same current draw .....read this somewhere but cant remember where .....

Fuel Pressure??

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