[V8] I can kind of see it

d saad dsaad at icehouse.net
Tue Feb 13 11:26:57 EST 2007

Fuel pressure and battery voltage are very critical for accurate fueling -
both for emissions and power/economy reasons.
So much so that many
new cars now have a fuel pressure sensor that can be monitored by the ECU.
Battery voltage is already monitored - even on our old systems. Both
factors are used to calculate injector pulse width. While the O2 sensor is
the final judge in injector pulse width, it has far less work to do if the
ECU chooses the right settings to start with - meaning there is less hunt
time (hysteresis) to maintain steady state conditions - like cruising, and
better accuracy for transient conditions like accell/decell. So the motor
spends more time making good power/economy while also making less

I think this is one of the reasons the V8 is so
prone to the "surge". It has a fairly primitive ECU, and the
feedback loop over corrects too quickly.


> I don't think that this is as important a variable on an
EFI-equipped car
> ... frankly, I've never considered adding it. 
There is some value in that
> it could tell you whether or not the
pressure regulator on the fuel rail
> was in need of replacement. 
I just replace the FP Reg if I still get rich
> mixture codes
after replacing the OXS ...
> On the CIS engines fuel
pressures were deliberately controlled to affect
> mixtures ... on
the V8 the intention is to keep the pressure differential
> across
the injectors constant so the ECU can set mixture with duty cycle.
> Steve B
> San José, CA (USA)
>> Has anyone installed a valve on the fuel rail to
measure fuel pressure?
>> I thought I read about it somewhere. 
The AF TWiki has one for S4/S6
>> cars.  It was an incredibly
valuable tool on my previous cars and I'd
>> like to have one
to check numbers.
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