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5.4L fuel pressure 40psi too much?

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Old 06-08-2010, 02:52 PM   #1
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Default 5.4L fuel pressure 40psi too much?

The spec is 28-45psi engine running...its at 40psi sitting at idle...i think thats too high what do you guys think?
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Old 06-08-2010, 03:01 PM   #2
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Default Not to Worry:

It's within specs. Don't fret.
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Old 06-08-2010, 03:08 PM   #3
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yeah well the truck is getting 13-14mpg...it has a CAI, 93 tune, flowmaster exhaust, new spark plugs...i should be getting 16 maybe 17mpg...2wd standard cab...and i don't drive like an idiot...i have a TT Mustang for that

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Old 06-08-2010, 03:41 PM   #4
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update for future reference for everyone..i was correct in thinkin it was too high..justin at VMP said it should be at 30psi at idle with vacuum line on...i adjusted it down and we shall see if my mpg increases
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Old 06-09-2010, 03:08 PM   #5
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Originally Posted by 98Lariat5.4L View Post
update for future reference for everyone..i was correct in thinkin it was too high..justin at VMP said it should be at 30psi at idle with vacuum line on...i adjusted it down and we shall see if my mpg increases
That's quite curious since it's not the injector or fuel pressure that determines the air/fuel ratio by themselves. They are part of the closed-loop system with the HGOS. If the fuel pressure is high and more fuel than necessary is injected, the rich mixture will be detected by the HGOS and tell the PCM to adjust injector on-time to compensate. Similarly the CAI is just that....cold air and its effect on the intake air temp sensor. Chilly conditions tell the PCM to inject a little more fuel to match the conditions.

I suspect the reduction of fuel mileage you have experienced may have more to do with the CAI and PCM strategy reprogramming than a seemingly high value of fuel pressure....anyway, keep us informed; knowledge is power.
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