4.6L Fuel Pressure Questions

Calvin099 · Dec 2, 2004

Just installed NOS Direct Port Noszel Wet kitfor my '99 GT vert. It's not kicking on because my fuel pressure is too low. At Idle I'm at about 32 psi with vaccum and 42 without.... This is a little low from published stock (35-45 with and 45-55 without I think). I'm not sure though if replacing the pump or pressure sensor is needed though... I would have not noticed the problem without installing the kit. Is there a way to change the fuel pressure setpoint? How do I get the pressure (with vaccum up to about 42 psi? What happens if I just run it without the vaccum? Thanks in advance...

blkhorse · Dec 2, 2004

I thought the 99's were returnless fuel system and don't rely on vacuum system(like in a return style)

YtnGT · Dec 2, 2004

blkhorse said:
I thought the 99's were returnless fuel system and don't rely on vacuum system(like in a return style)

Wrong... (They use vacuum but not in the same way 98 & previous cars did.)
I have posted this info before but here goes. 99+ mustangs use the computer as the fuel pressure regulator. The fuel pressure sensor is referenced to manifold absolute vacuum. In other words you are always maintaining 39psi across the injector.

Please read here for more info!!!!!!!!!!!!!!!

YtnGT · Dec 2, 2004

Also, I have a FPSS (fuel pressure safety switch) on my car. It can be adjusted, mine opens (open circuit = no N2O) up around 32 psi. I used air pressure to test it.

It should have been set from NOS at approx 35psi ?????

blkhorse · Dec 2, 2004

the computer uses a lot of info(tps.iat,mass air,cts,fuel temp,manifold pressure etc) to determine what to tell the fuel pump module driver what to set pump voltage at,I don't think just unplugging the vaccum line will do a lot except set a code,then the computer will substitute a known value and put you back where started

YtnGT · Dec 2, 2004

blkhorse said:
the computer uses a lot of info(tps.iat,mass air,cts,fuel temp,manifold pressure etc) to determine what to tell the fuel pump module driver what to set pump voltage at,I don't think just unplugging the vaccum line will do a lot except set a code,then the computer will substitute a known value and put you back where started

The computer uses the fuel pressure sensor to determine what duty cycle to set the PWM (pulse width modulation) output at.

Unplugging the vacuum line will cause the fuel pressure to always be 39psi referenced to atmosphere. The delta pressure across the fuel injector will be higher at idle and cause drivability issues and a MIL.

Read the post I linked to previously and apply a little physics and you should understand what is going on.

Calvin099, don't replace the pump computer or anything else. Don't run the car with the fuel pressure sensor vacuum line off. Check the FPSS set point, use compressed air, a regulator, and a DVM to check continuity.

Good Luck & let us know how the NOSZLE system works out.

Calvin099 · Dec 2, 2004

YtnGT said:
The computer uses the fuel pressure sensor to determine what duty cycle to set the PWM (pulse width modulation) output at.

Unplugging the vacuum line will cause the fuel pressure to always be 39psi referenced to atmosphere. The delta pressure across the fuel injector will be higher at idle and cause drivability issues and a MIL.

Read the post I linked to previously and apply a little physics and you should understand what is going on.

Calvin099, don't replace the pump computer or anything else. Don't run the car with the fuel pressure sensor vacuum line off. Check the FPSS set point, use compressed air, a regulator, and a DVM to check continuity.

Good Luck & let us know how the NOSZLE system works out.

The fuel safety switch was set way high... Up around 45 psi... I've lowered it to 37 (called NOS, this is what they said to use) but it still seems high.... Any comments on what to set the FPSS at? Is 32psi the norm?

YtnGT · Dec 2, 2004

Calvin099 said:
The fuel safety switch was set way high... Up around 45 psi... I've lowered it to 37 (called NOS, this is what they said to use) but it still seems high.... Any comments on what to set the FPSS at? Is 32psi the norm?

Is 37psi the pressure at which the switch opens? Did you set it with air pressure? Remember there is a slight difference in the opening and closing pressures.

When the fuel solenoid first opens it will cause a brief & slight drop in fuel pressure, as the computer sences the drop in FP it will add duty cycle to the fuel pumps to bring the pressure back up. You don't want this brief drop in FP to trip the FPSS but you don't want the pressure to drop to much before the FPSS kicks the N2O system out. Remember low fuel pressure = lean = new motor.

I have a wideband a/f meter to keep check on the a/f ratio, I only use the FPSS for (added) protection against pump failure, pump wiring failure, etc...

I would put a DVM on the FPSS while it is on the car. If it is open when at idle and closed at WOT (blip the throttle body) then you are probably OK. I woud take it to a dyno with a/f for your first run or buy an a/f meter $350-$500. Just to make sure.

Calvin099 · Dec 2, 2004

I haven't put it on a dyno yet, I need a new tune anyways after all the mods I made. But I did buy a fuel pressure autometer for the car. I was gonna hook it up this weekend and try to engage the NOS and watch pressure. I tested the electronics on the NOS system tonight with a voltmeter with a WOT and the FPSS..... it seems to go high every time..... Took the car on the road and got it to engage 2 out of 4... I think I need to lower the FPSS a little more...... Maybe to your 32 psi level..

YtnGT · Dec 2, 2004

Yea if it engages 2 out of 4 times then you are probably right on the border line. That is good because at least you know where you are at. I would probably try it several more times without readjusting the FPSS, if it continues to work intermitently then lower the FPSS set point a little more. It is better to be on the safe side. If your N2O cuts out on a run because your FPSS is set to high, who cares. Lower the set point a little and try it again. Get greedy, reckless, or in a hurry with it and get a new engine.

Calvin099 · Dec 3, 2004

Sidenote: I noticed some concerns around running a predator and nitrous at the same time..... Mainly around timing. Can I still use a predator tune on my car or should I go and get a custom tune? I already have the Predator for my stang but took it off when I installed the NOS kit.

lopey4.6 · Dec 3, 2004

The kit is designed to work with the stock computer settings. I would take the predator off. Also I decided to take the FPSS off my kit because I had the intermittent problem your are expierencing. I have installed a pillar mount fuel pressure gauge, and have a AEM pillar mount wide band O2 ready to go in.

Also my fuel pressure reads about 32 and about 40 at WOT, then goes to 75 while nitrous is engageg (I have a dry kit with a aviator pump) Hope that helps.

YtnGT · Dec 3, 2004

Calvin099 said:
Sidenote: I noticed some concerns around running a predator and nitrous at the same time..... Mainly around timing. Can I still use a predator tune on my car or should I go and get a custom tune? I already have the Predator for my stang but took it off when I installed the NOS kit.

You can use the predator but don't use the predator performance tune. The predator perfornance tune advances the timing and leans out your a/f (especially at rpms lower than 4000).

I would recommend using the "factory HP tune"... Then you can add +5 WOT fuel and retard the timing some (5-20 units). That will make things even safer than stock but you will loose some HP. But who cares you have the NOSZLE!!!!

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