i just put a fuel pressure guage on the fuel rail a few weeks ago and noticed that the pressure is not staying constant. i will crank it one time and its at 30psi then the next day it may be at only 20psi. i figured the AFPR would fix this problem when i got around to putting it on but it didnt, could this be the fuel pump or should i look somewhere else for the problem?
Possible bad fuel pump???
- Thread starter fiveohh94gt
- Start date
mo_dingo
20+ Year Stangneter 
Are you testing the fuel pressure with, or without vacuum? You need to disconnect the FPR's vacuum source, so it won't lower the fuel pressure.
I got 28psi w/ vacuum, and 36 w/o vacuum (I think). Post both numbers.
Remember, the FPR's purpose is to regulate fuel pressure when the throttle is closed b/c the extra vacuum will suck too much fuel out of the injectors. It also regulates MAX pressure when no vacuum is present.
If there is a problem, the FPR or the fuel pump could be bad. There is a test in the haynes manual that will tell you how to find out for sure. (Note: You won't find any rubber, squeezeable fuel lines under the hood. You need to look where the fuel lines start, at the gas tank.
Scott
I got 28psi w/ vacuum, and 36 w/o vacuum (I think). Post both numbers.
Remember, the FPR's purpose is to regulate fuel pressure when the throttle is closed b/c the extra vacuum will suck too much fuel out of the injectors. It also regulates MAX pressure when no vacuum is present.
If there is a problem, the FPR or the fuel pump could be bad. There is a test in the haynes manual that will tell you how to find out for sure. (Note: You won't find any rubber, squeezeable fuel lines under the hood. You need to look where the fuel lines start, at the gas tank.
Scott
when exactly is the FP reading the numbers you quote? is that the initial reading after priming, and then does the pressure go up after you start the car and it is idling? remember there is a check valve to maintain pressure. does pressure go up more if you cycle the key a couple times before starting?
stuff like that, and what Scott asked, will help.
right now, if i had to guess, i would say your pump is on the way out. i would think you have eliminated the FPR with the new one, as a culprit.
my ramblings. good luck.
stuff like that, and what Scott asked, will help.
right now, if i had to guess, i would say your pump is on the way out. i would think you have eliminated the FPR with the new one, as a culprit.
my ramblings. good luck.
stang_gt_1994
Founding Member
the #'s i said were both with the engine running and warmed up. i've done the test in the haynes manuel and it checks out fine, but that test only tells you if its pumping at all, not if its not working at full potential, and yes it is a stock pump probably time to upgrade anyway, what size pump would i need?
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