If I raise or lower fuel pressure will it affect the air fuel ratio or does the computer adjust it back?

 

well, the computer cannot tell what the fuel pressure is but if it sees a rich condition it will make an adjustment to compensate. But the computer can only do minor adjustments. mabye 3, 5, 7 PSI but im not sure exactly how much the computer can do.

 

The PCM can not alter the fuel pressure in any way, shape or form. It has no way of even knowing the fuel pressure.

What happens when you change the fuel pressure.... as an example, if you increase the fuel pressure, for a given pulse width the injector will deliver more fuel than the PCM assumes it is delivering. The O2 sensor sees the "rich" condition, and the PCM starts to reduce the long term fuel corrections below the "standard" value of 128. Reducing the long terms, also known as block learn multipliers (BLM's), has the effect of shortening the pulse width - the time the injector is held open. In effect, raising the fuel pressure causes the injector to deliver more fuel, so the PCM just keeps it open for a shorter period of time, to deliver the exact same amount of fuel it was delivering before you made the pressure change.

The PCM can "add" up to 25% more fuel to correct a lean condition using the BLM's, can "cut" up to 15% of the fuel to correct for a rich condition.

There is a general belief that the responsiveness of the PCM is not quite as great with the 93's, as compared to the 94-97 models, so the answer depends partly on the year of your car.

 

Thanks for the replies, I do have a 93 but it appears if I have an air fuel ratio on the dyno that I don't like, I'm better off having the chip burned to compensate. I have a Grand National and we tune alot with fuel pressure.