Originally Posted by 98Lariat5.4L
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.