Can Fuel Pressure Be Set With Engine Off?

[tsi1991awd]

I have always set fuel pressure with the motor on and the vacuum hose disconnected and plugged with my finger, to get base pressure.

But can it be set with the motor off and using the fuel pump check by the battery? My base pressure will be quite low upon first start up if I set it with the motor running, since the new AFPR install just went underway and I'm just now getting my chip, etc.

Any thoughts?

 

[Perkul8r]

Just read the VFAQ and it says to just leave the vacuum line off the afpr and use the fuel pump check terminal to pressurize the system and set the fuel pressure.

 

[1gengsx]

Yes you can I set mine key on engine off. With out any problems.

 

[delta448]

You can usually get it close enough, but battery voltage does effect the fuel pump, so setting it while the alternator is in use is better.

 

[tsi1991awd]

All I want to do is get a ballpark figure so I can start the car without it being way too low of pressure. I can adjust it once the car is set....but thanks for the clarification. I'll set the pressure with the car off first

 

[steve]

Yes you can set it by powering the pump from the test connector or using DSMLink if you have it.

Once the AFPR is set to the right pressure and assuming it's able to handle the cars fuel pump it won't make any difference how much more voltage the pump receives. 36.3 psi at the regulator is always 36.3 psi as long as the regulator is working.

 

[tsi1991awd]

43.5psi for me

 

[PieEyedPiper]

Just for clarity, is it correct that a 1g enables the fuel pump in key on/acc. position and the 2g will not until you crank it?

 

[Red97Eclipseboy]

Just for clarity, is it correct that a 1g enables the fuel pump in key on/acc. position and the 2g will not until you crank it?

I think all turbo DSMs don't unless you're cranking.

 

[steve]

Just for clarity, is it correct that a 1g enables the fuel pump in key on/acc. position and the 2g will not until you crank it?

Nope.

I think all turbo DSMs don't unless you're cranking.

Some cars seem to have a glitch coming from their position sensors on power up that cause the ECU to think the crankshaft is moving, the ECU to turn the pump on and then time out when it doesn't see the pulses continuing. From what I can tell reading the code and the way the MPI relay is designed that the fuel pump shouldn't turn on until you start cranking the engine. This is the same on both 1G's and 2G's. The 2G ditched the second input from the starter circuit to the MPI/fuel pump relay but that was somewhat redundant anyway.

 

[TenSecondSTi]

also theres no vac present. without it running so your FP will seem less

 

[91tizzy]

thats why you pull the vacuum line off the regulator dude

 

[steve]

also theres no vac present. without it running so your FP will seem less

thats why you pull the vacuum line off the regulator dude

If your setting it with the engine off there is no vacuum in the intake manifold, it's at atmospheric pressure so the reference port of the FPR sees exactly the same pressure as if the hose was disconnected.

Once you start the car the vacuum at the reference port will lower the fuel pressure but the base pressure remains the same. The pressure difference between the fuel rail and intake manifold is maintained at the base fuel pressure as long as the regulator tracks 1:1.

 

[delta448]

Steve, have I been misguided that an adjustable regulator would need to be readjusted for a change in pump output?

I guess I've not used one in such a long time that I've forgotten things.

 

[steve]

The pump has to produce enough flow off the battery even during cranking to pressurize the fuel system. This is a requirement.

The fuel pressure regulator has to be able to regulate the pressure under full pump flow and max voltage. This is also a requirement.

As long as both requirements are met then there should be no change in the regulated pressure across the full range of system voltages. And as long as the FPR functions correctly and tracks the pressure at the reference port the injectors should always deliver the same flow rate per unit of time regardless of voltage and manifold pressure.

The typical places where these conditions fail are the stock FPR that can't handle the flow of a upgraded pump, pumps that can't keep up with upgraded injectors, etc.

 

[mnetwork]

You can usually get it close enough, but battery voltage does effect the fuel pump, so setting it while the alternator is in use is better.

I was thinking about this the other day and this is what I was thinking...

Yes you can set it by powering the pump from the test connector or using DSMLink if you have it.

Once the AFPR is set to the right pressure and assuming it's able to handle the cars fuel pump it won't make any difference how much more voltage the pump receives. 36.3 psi at the regulator is always 36.3 psi as long as the regulator is working.

... BUT THEN I was thinking this. An AFPR is made to keep fuel pressure at a certain pressure NO MATTER what the fuel pressure and flow is behind it (unless the pump is flowing so low it can't keep up, this shouldn't be an issue at base fuel pressure unless the battery was very low)...

This would be like saying a boost controller is going to be accurate when you are at 4k, but not at 7k. The only time that would be the case is if there was an issue with the wastegate or if it was a smaller turbo and it couldn't keep up with the motor...