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Old 05-14-2010, 04:02 AM   #1
redmaro1
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Exclamation Low FUel Pressure. Regulator or Fuel Pump?

So the Mustang GT 1998 seems to hesitate once in a while and when ever I go half throttle or so it takes a while to respond. So I checked the fuel pressure at idle and it reads 18 psi. I unhooked the vacuum line to the fuel pressure regulator and the pressure did not go up (Now fuel in vacuum lines btw). Right away I thought the problem was the fuel pressure regulator. But after thinking for a while it got me thinking. Wouldn't a faulty low pressure fuel pump cause the regulator to not raise fuel pressure once the vacuum line has been disconnected since the fuel pump pressure is low and there's not enough fuel pressure to make the regulator work properly?

What do you guys think? Faulty Fuel Pump or Fauly FPR?



Also, whats should the stock fuel pressure be for an 98 GT at idle?

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Old 05-14-2010, 04:49 AM   #2
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This is from the 2000 shop manual, your '98 should be the same:

Click the image to open in full size.

These are the reference vaues for the fuel rail pressure sensor output voltage (FRP V), and the fuel rail pressure (FRP) in psi.

KOEO = Key On Engine Off
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Old 05-14-2010, 05:11 AM   #3
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I haven't checked the Fuel pressure with the engine off and key on. If it reads low with the engine off would that rule out a bad fuel pressure regulator?
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Old 05-14-2010, 05:43 AM   #4
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Quote:
Originally Posted by redmaro1 View Post
I haven't checked the Fuel pressure with the engine off and key on. If it reads low with the engine off would that rule out a bad fuel pressure regulator?
I may have led you astray a bit, as I just now looked in the limited '96 to '98 sources and find that the Cobra (and maybe the GT too) have return type fuel systems--I think I know that in the back of my head somewhere but it seemd to get stuck...

Because of that the fuel pressure specs I listed are likely incorrect as they are for the returnless system, which provides much more precise pressure regulation.

In answer to your question, in a retun type system low pressure at idle would be indicative of a bad regulator, or fuel pump. Perhaps someone with a '98 shap manual can provide the pressure specs for your car.
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Old 05-14-2010, 09:49 AM   #5
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i have a chilton manual ill post the numbers up here a little later im not home right now.
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Old 05-14-2010, 09:49 AM
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