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Old 10-26-2009, 02:52 PM   #1
03SCGT
 
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Default Testing Fuel Pressure

Just picked up an 03 GT with a Vortech V2 supercharger setup. The car appears to be done correctly with all the right components other than larger injectors than what came with the setup. On the dyno it was running way too lean so we need to hook up a fuel pressure gauge and watch the gauge while driving to see if the pressure goes downhill.

How/where do I get the correct components to hook up a fuel pressure gauge to this car? The quick disconnect is easy enough to take apart but finding a suitable FP guage/t-fitting is proving to be tough. I was kind of surprised to not see a schrader valve to hook into.

Also, any other suggestions regarding my lean condition would be appreciated.

Thanks guys.
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Old 10-26-2009, 06:38 PM   #2
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you have an '03 so you should have a schrader valve on the pass. side fuel rail
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Old 10-26-2009, 11:30 PM   #3
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Originally Posted by Brute03 View Post
you have an '03 so you should have a schrader valve on the pass. side fuel rail
It went away someplace along the line, though I have not pinned down exactly when--my '03, made in February of '03, does not have it.

If you have access to an OBD2 data logger you can read fuel pressure with PID $010A...
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Last edited by cliffyk; 10-27-2009 at 12:01 AM.
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Old 10-27-2009, 05:00 PM   #4
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It went away someplace along the line, though I have not pinned down exactly when--my '03, made in February of '03, does not have it.

If you have access to an OBD2 data logger you can read fuel pressure with PID $010A...
Great idea on the data logger. We hooked up a diablo tuner for a 03 Cobra which worked great for watching the fuel pressure as we drove around. The fuel pressure was around 40psi at idle and while driving around normally. When I put my foot into the gas, the fuel pressure stayed around 38-41 and didn't go up at all, hence my lean problem.

Now here's where I don't understand how Mustangs work. The fuel pressure regulator is just that little chrome silver unit with NO vacuum line going to it. Without a vacuum line at the regulator, how/why does a normal Mustang get it's fuel pressure to go up to 60psi at WOT? We hooked the Diablo up to the 03 Cobra and the fuel pressure shot up to 60psi as soon as we got on the gas.

I'm trying to understand how the Mustang fuel system works and what makes the fuel pressure go up to 60psi at WOT ???

Last edited by 03SCGT; 10-27-2009 at 05:03 PM.
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Old 10-28-2009, 08:26 AM   #5
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What you have data logged is not your real fuel pressure. Your real fuel pressure may have gone to 60 psi. The computer corrected number stayed between 38-40 psi. Our cars do not have a fuel pressure regulator. On the driver's side fuel rail is a fuel pressure sensor that sends signals to the computer. Our car's returnless system also has a variable speed fuel pump that increases or decreases fuel pressure as needed.

I have an after market fuel pressure sensor mounted on the passenger side fuel rail. It measures the real fuel pressure. The only time that it and the corrected fuel pressure agree is when I turn the key on but do not start the car. They both read 40 psi.

You can do a rough calculation to find the real fuel pressure. You subtract vacuum divided by two from 40 or add boost to 40. For example, I have about 18 in. hg. (inches of mercury) of vacuum at idle. 40 - (18/2) = 31 psi of real fuel pressure. My after market gage confirmed this. It normally reads 30 psi at idle. At WOT near 6000 rpm my supercharger is producing 8 lbs of boost. 40 + 8 = 48 psi real fuel pressure. My after market gage swings up to near 50 psi at this time. I have never seen my gage go to 60 psi. I would need approximately 20 lbs of boost to get it there.

The computer will also increase the real fuel pressure on a hot day to prevent vapor locking in the fuel rails. It uses the inlet air tempature to approximate fuel rail tempature. I have seen my after market gage read as high as 50 psi at idle in stop and go traffic. The data log said 40 psi.

If you want to monitor your real fuel pressure you need to get an after market fuel pressure gage. But don't get worried if you see the fuel pressure needle on that gage sweep up and down. That is perfectly normal.
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Old 10-28-2009, 09:36 AM   #6
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fwiw my data logged f/p ALWAYS reads higher than the actual rail pressure, usually by 10-15 psi
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Old 10-28-2009, 10:41 AM   #7
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It should. Unless you're running a supercharger (or turbocharger) you would never get into a boosted situation and, even when accelerating, our cars normally show a vacuum. The only time that you would see the real fuel pressure higher than your data log is on hot days. Cruising down the highway at 80 mph my after market fuel pressure gages reads around 34-35 psi. If I were to data log it, the computer corrected number would read 40 psi.
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Old 10-28-2009, 04:13 PM   #8
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It (the f/p reported by PID $010A) is the real fuel pressure, at least the only one that matters as far as tuning is concerned, as it is the pressure drop across the fuel injectors--which of course affects how much fuel they inject--I.e. their effective capacity.

The PCM (unless told otherwise in the tune) will always assume that the pressure across the injectors is 40 psi, and calculate a pulse width based on that assumption.
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Old 10-30-2009, 01:52 PM   #9
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Sematics. What should I have called the fuel pressure at the fuel rails? Maybe raw fuel pressure would have been a better term.

I understand that it is the differential fuel pressure that allows the injectors to squirt in the correct amount of fuel. But this got me thinking. At idle if the fuel pressure at the fuel rails is 30 psi and we have 18 inches of vacuum then the differential pressure across the injectors is approximately 40. That's normal. That's what you data log. What happens on hot days when the fuel rail pressure is 50 psi at idle? Vacuum is still 18 inches. Wouldn't that make the differential across the fuel injectors around 60 psi?

I had my car data logged under those exact conditions and the data log said 40-43 psi.
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Old 10-30-2009, 04:33 PM   #10
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Quote:
Originally Posted by Eagle2000GT View Post
Sematics. What should I have called the fuel pressure at the fuel rails? Maybe raw fuel pressure would have been a better term.

I understand that it is the differential fuel pressure that allows the injectors to squirt in the correct amount of fuel. But this got me thinking. At idle if the fuel pressure at the fuel rails is 30 psi and we have 18 inches of vacuum then the differential pressure across the injectors is approximately 40. That's normal. That's what you data log. What happens on hot days when the fuel rail pressure is 50 psi at idle? Vacuum is still 18 inches. Wouldn't that make the differential across the fuel injectors around 60 psi?

I had my car data logged under those exact conditions and the data log said 40-43 psi.
The PCM makes adjustments to the rail pressure according to its observations of a number of settings, IAT and ECT being two--when the IAT is hot it raises the rail pressure to enrichen the mix and cool combustion chamber temps--this reduces emissions, NOx mostly, and the tendency to ping at the higher intake and cc temps.

Making the rail pressure 50 psi, rather than 40, makes the 21 lb/h injectors behave like 23.5 lb/h injectors--a quick and easy way of dealing with the hig intake and engine temps...
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Old 10-30-2009, 04:33 PM
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