Fuel pressure at idle?

[Big Top Gt]

I pulled the oil pressure and water temp gauges from my 05 and replaced them with both intake air temp and fuel pressure gauges.

It occurred to me while driving the car this afternoon that I don't know what the fuel pressure should be at idle. Right now, assuming the gauge is reading properly, it's showing 30 PSI at idle and roughly 40 when I try to get the car moving.

Since I haven't had the car tuned for the Saleen blower yet, I keep the car out of boost, so I don't know if the fuel pressure climbs to more than 40 PSI.

In my Cobra I know the fuel pressure should be roughly 40 psi at idle, but I'm not sure that number translates over to the 05.

Thanks for the help,
Alan

 

[thump_rrr]

I pulled the oil pressure and water temp gauges from my 05 and replaced them with both intake air temp and fuel pressure gauges.

It occurred to me while driving the car this afternoon that I don't know what the fuel pressure should be at idle. Right now, assuming the gauge is reading properly, it's showing 30 PSI at idle and roughly 40 when I try to get the car moving.

Since I haven't had the car tuned for the Saleen blower yet, I keep the car out of boost, so I don't know if the fuel pressure climbs to more than 40 PSI.

In my Cobra I know the fuel pressure should be roughly 40 psi at idle, but I'm not sure that number translates over to the 05.

Thanks for the help,
Alan

Fuel Pressure Differential should be a constant 39psi.

So to measure the fuel pressure differential in vacuum the formula is Inches of Hg / 2.036 = psi.
Assuming that you have 22" Hg

22 inches Hg / 2.036 = -10.84 psi
so 39 - 10.8= 28.2psi
while in boost you would just add boost pressure to the constant 39psi. so at 10lbs of boost you should have 49psi.

 

[Kevin@PMP]

http://www.s197forum.com/forum/showpost.php?p=154698&postcount=3

 

[Big Top Gt]

Thanks guys.

I don't really get the tuning end of these things at all. I suck.

 

[Big Top Gt]

From the other thread:

If I need to use vacuum readings to figure out fuel pressure, what's the point of putting a Fuel Pressure gauge in these cars?

It doesn't make sense.

I've got a boost/vacuum gauge in the car. I just walked out to the car and started it. It's showing 29-30 pounds of fuel pressure and roughly 20-22 in (hg?) of vacuum. (per the gauge)

Which falls right into line with your math above. This setup is different than the one in my Cobra. With that fuel setup (return style) you pull the vacuum hose, set it to 39, and reattach the vacuum line. Voila!!! Fuel Pressure is set. With the returnless setup, which is still uncharted territory for me, there's no pulling the line.

Thanks! I appreciate the help.

Oh, and for those of you who are wondering what "Delta P" is; it means Change in pressure, in this case across the injectors. Hope this helps.

 

[thump_rrr]

From the other thread:

If I need to use vacuum readings to figure out fuel pressure, what's the point of putting a Fuel Pressure gauge in these cars?

It doesn't make sense.

I've got a boost/vacuum gauge in the car. I just walked out to the car and started it. It's showing 29-30 pounds of fuel pressure and roughly 20-22 in (hg?) of vacuum. (per the gauge)

Which falls right into line with your math above. This setup is different than the one in my Cobra. With that fuel setup (return style) you pull the vacuum hose, set it to 39, and reattach the vacuum line. Voila!!! Fuel Pressure is set. With the returnless setup, which is still uncharted territory for me, there's no pulling the line.

Thanks! I appreciate the help.

Oh, and for those of you who are wondering what "Delta P" is; it means Change in pressure, in this case across the injectors. Hope this helps.

It is no different from an 03-04 terminator. The Fuel Rail Pressure Sensor or FRPS on the drivers side fuel rail has fuel pressure on its lower side and vacuum or boost on the top side. If you pull the vac hose off of the FRPS you should see 39psi on your gauge.
You can also just turn the key to position 2 and see that the pressure goes to 39psi.

 

[Big Top Gt]

It is no different from an 03-04 terminator. The Fuel Rail Pressure Sensor or FRPS on the drivers side fuel rail has fuel pressure on its lower side and vacuum or boost on the top side. If you pull the vac hose off of the FRPS you should see 39psi on your gauge.
You can also just turn the key to position 2 and see that the pressure goes to 39psi.

My Cobra's a 98 and there weren't any FRPS' on a 98.

Good tip. I'll check it out later today.

 

[Black Tooth Grin]

Fuel Pressure Differential should be a constant 39psi.

So to measure the fuel pressure differential in vacuum the formula is Inches of Hg / 2.036 = psi.
Assuming that you have 22" Hg

22 inches Hg / 2.036 = -10.84 psi
so 39 - 10.8= 28.2psi
while in boost you would just add boost pressure to the constant 39psi. so at 10lbs of boost you should have 49psi.

http://www.s197forum.com/forum/showpost.php?p=154698&postcount=3

As many times as this information has been asked and posted, maybe someone should suggest it be a sticky. Provided that anyone paid any more attention to stickys than the do the search button.

 

[Big Top Gt]

As many times as this information has been asked and posted, maybe someone should suggest it be a sticky. Provided that anyone paid any more attention to stickys than the do the search button.

What would be better would be if when you searched for the term "fuel pressure" this information would come up.

When I did that last night the only one that was close was a thread with a title suggesting it someone was looking to pull the info out of the ECU. Since that wasn't what I was looking for, I never even clicked it until ThumpRRR linked to it.

Now it does, so maybe a sticky isn't necessary.