Fuel Pressure Q??

Discussion in 'Fox 5.0 Mustang Tech' started by RobSSon, Jun 24, 2008.


  1. RobSSon

    RobSSon New Member

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    Hi everyone! Obviously ths is my first post, and I want to thank all of you for the information that I have obtained from this site:SNSign:. Here is my issue, hopefully someone can give me a detailed explanation to help me resolve my problem:

    I recently bought an 89.5 mustang gt. It has a 331 stroker, AFR 185's, 42 lb injectors, Trickflow intake, Accufab throttle body, and other odds and ends. The car was almost impossible to drive when I first bought it. I have tuned it to the point that it behaves for the most part. Originally the fuel pressure was set at 42 psi with a Holley AFPR, and the timing was set at 16 degrees. The car was very powerfull, but misfired from 3750 rpm and up. I knocked down the timing to 14 degrees, and lower the fuel pressure to 38 psi. The car is lower on power, but it doesnt sputter any more. Obviously, I need to get a dyno tune, but I can't take any time off of work for at least 3 months to accomplish this. My first question might be a dumb one, but I want to double check. At idle, with no vacuum, the fuel pressure should read around 38 psi. With the vacuum line in, the pressure should drop a little (1-3 psi). Is the fuel pressure supposed to drop even lower at WOT?:shrug: Mine drops to about 30 psi... Another dumb question, should I run the vacuum line that goes to the FPR from the upper intake, or should I run it from the main vacuum T on the fire wall. The reason I ask this is because the vaccum from the upper intake only measures about 7 psi, and drops at WOT, but the vacuum on the main T measures 14 psi at idle, and about 24 psi at WOT. Take into consideration that I have checked for vacuum leaks ( all hoses are new and connected correctly), but the car has a pretty large cam (562 int/572 ex). I'm having a major brain fart.... I jsut want to make sure that the FPR is operating correctly. I don't really trust the Holley FPR, and I was thinking of getting a Kirban. Thanks for your help!
  2. mustangfan1990

    mustangfan1990 New Member

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    At WOT there is no vaccuum. 42 pounds injectors are aweful big for an all motor car. The sputter that you are experienceing I would uess would be flooding considering you are taking out timing and fuel. The only way I see you fixing your problem and getting the most from that engine is a custom tune no ways around it


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  3. RobSSon

    RobSSon New Member

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    Thanks for the reply! I know the 42lbers are too big. I have a paxton coming in the mail next week, so I figured it would be a waste of time swaping them out. I'm talking back and forth with Bob Kurgan to try to get some time set aside for a tune. Hopefully that works out.

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