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Discussion Starter #1
So I just put on some 24 lb injectors and a lightning maf with a tune, and I've been seeing my fuel pressure rise steadily while driving. On a cold start, it'll be right at 40, after it heats up it'll be around 45, and if I'm in stop and go traffic or driving light to light it'll go up as high as 55. That seems a little high to me, what could cause that rise?
 

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US Air Force (retired)
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Are you talking about a fuel pressure reading at the fuel rail or are you talking about the differential pressure across the face of the injector? I'm assuming that you are reading an On-Board Diagnostic from the ECU since a fuel pressure gauge mounted on the fuel rail never reads exactly 40. It reads 40 minus Vacuum/2 or 40 plus boost. As the gas pedal moves the needle on fuel rail pressure gauge swings up and down. At idle with vacuum reading 18 in.hg. my fuel pressure is 30-31 psi at the fuel rails (40 psi - 18/2)

There is one exception and this may be the answer. You are in Florida so its a lot warmer there than it is here. Based on inlet air temperature and other parameters the ECU will raise the fuel pressure and shorten the pulse to prevent vapor locking in the fuel rails. My normal fuel pressure at the fuel rails is 30 psi at idle. On a warm day in stop and go traffic I have seen it as high as 50-55 psi. If the injector pulse had remained the same I doubt the motor would have run smoothly. It would have been getting way too much fuel. But when I get on the open road needle comes back down to normal.

It took me the purchase of a new FRPS and $300 of troubleshoot at a local Ford dealership to learn that.
 

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Discussion Starter #3
Are you talking about a fuel pressure reading at the fuel rail or are you talking about the differential pressure across the face of the injector? I'm assuming that you are reading an On-Board Diagnostic from the ECU since a fuel pressure gauge mounted on the fuel rail never reads exactly 40. It reads 40 minus Vacuum/2 or 40 plus boost. As the gas pedal moves the needle on fuel rail pressure gauge swings up and down. At idle with vacuum reading 18 in.hg. my fuel pressure is 30-31 psi at the fuel rails (40 psi - 18/2)

There is one exception and this may be the answer. You are in Florida so its a lot warmer there than it is here. Based on inlet air temperature and other parameters the ECU will raise the fuel pressure and shorten the pulse to prevent vapor locking in the fuel rails. My normal fuel pressure at the fuel rails is 30 psi at idle. On a warm day in stop and go traffic I have seen it as high as 50-55 psi. If the injector pulse had remained the same I doubt the motor would have run smoothly. It would have been getting way too much fuel. But when I get on the open road needle comes back down to normal.

It took me the purchase of a new FRPS and $300 of troubleshoot at a local Ford dealership to learn that.
I'm referring to the delta FP, I'm watching it via a handheld, not the physical pressure at the rail. I think with sct they call it the "pressure drop across injectors". I wasn't sure if that corresponded to the physical pressure. I'll keep an eye on it next time i drive, and I'll try checking the physical pressure at start up and at temp to see if its as high as I was assuming.

If that's pretty normal in traffic I won't worry about as much, I never watched the fuel pressure before I swapped the injectors so I wasn't sure what to expect.
 
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