fuel pressure question

[98gtmustang64]

i got a 98 gt and i was wondering what stock fuel pressure is cuz i think mines low i just tested it and it said 30 psi at idle and if u stab the throttle it jumps to like 39 then falls back down to 30. Then when i shut the car off it stayed at 20 idk if this is right or what.

 

[sneaky98gt]

i got a 98 gt and i was wondering what stock fuel pressure is cuz i think mines low i just tested it and it said 30 psi at idle and if u stab the throttle it jumps to like 39 then falls back down to 30. Then when i shut the car off it stayed at 20 idk if this is right or what.

Sounds right. The falling to 20 is probably because the pump cuts off and it loses a certain amount back through the return line.

 

[98gtmustang64]

so everything seems normal then

 

[sneaky98gt]

I think so. The 30 at idle and 39 at WOT is exactly what my car does. Mine falls a little when I cut it off (around 28-29), but hasn't ever been a problem. 20 seems a little low to fall down to, but I'll let someone else with more knowledge that me chime in to say for sure.

 

[98gtmustang64]

k cuz im running a zex wet kit and wanted to make sure everything is how its suppose to be cuz when i run the 100 shot its done this twice now it has blown the air filter off i didnt no if that was a fuel problem or not

 

[98COBRA281]

39 psi at idle, with the vacuume line off the regulator.

 

[98gtmustang64]

so i need to take the vacum line off and check it again cuz i did it with it on

 

[wmburns]

It might help if you understood what the vacuum line does. It provides a reference point for the fuel pressure.

The external guage measures atomspheric. So by disconnecting the vacuum line, the regulator is also operating atomspheric.

The fuel pressure is susposed to 40 PSI above the intake manifold pressure. At an idle, the intake is under vaccuum. To correctly compensate, the fuel pressure is "referenced" to the lower vacuum pressure (40-10=30).

Under a boosted application, the intake manifold is under pressure. Let's say the boost is 10 PSI. Then the fuel pressure should be 50 PSI (40+10=50).

What many ppl do for a quick test is to disconnect the vaccuum line. The fuel pressure is now atomspheric referenced and should be 40 PSI (for a Mustang).

Do not drive the car with the vacuum reference line disconnected. It will mess up the AF ratio. Under boosted applications, motor damage is likely.

 

[98gtmustang64]

so with the vacum line on its 30 and with it off its 40 correct? and who knows why my nitrous might have back fired through my intake

 

[sneaky98gt]

It might help if you understood what the vacuum line does. It provides a reference point for the fuel pressure.

The external guage measures atomspheric. So by disconnecting the vacuum line, the regulator is also operating atomspheric.

The fuel pressure is susposed to 40 PSI above the intake manifold pressure. At an idle, the intake is under vaccuum. To correctly compensate, the fuel pressure is "referenced" to the lower vacuum pressure (40-10=30).

Under a boosted application, the intake manifold is under pressure. Let's say the boost is 10 PSI. Then the fuel pressure should be 50 PSI (40+10=50).

What many ppl do for a quick test is to disconnect the vaccuum line. The fuel pressure is now atomspheric referenced and should be 40 PSI (for a Mustang).

Do not drive the car with the vacuum reference line disconnected. It will mess up the AF ratio. Under boosted applications, motor damage is likely.

My car is boosted at 6psi. You're saying it should be 46 psi with the vacuum line off? I just went and pulled it off, and it's on 40 at the dot. If it should be 46, then I may need to get that checked out by my tuner.

And for whatever reason, it doesn't always go to 40 at WOT. Normally 38ish. Depends on how long I'm on it. A long, drawn out 2nd gear pull and it will sometimes drop to 36ish. Is that normal either? After a few things other people with similar setups as me have told me, I'm beginning to question just how good of a job my tuner did.

so with the vacum line on its 30 and with it off its 40 correct? and who knows why my nitrous might have back fired through my intake

Wet or dry shot? If that was a wet shot, what RPM did you spray? Low RPM wet shot = fuel puddling in the intake = KABOOM to the intake. Generally speaking not a good idea to do. I'd definitely figure out what is going on before spraying it any more.

 

[98gtmustang64]

its a wet kit and its happened at high rpms not low