Fuel injectors are designed (rather, physics has made them design fuel
injectors) to work at a specific pressure drop across the injector
nozzle...
This is designed to give the optimum spray pattern for the fuel pressure,
nozzle shape, orofice size and probably many things my non-engineering
background is overlooking. Wander too far from this pressure and it can
affect the spray pattern and/or atomization of the fuel. (Both of these
are
critical for proper combustion and cylinder dispersion of the fuel.
This is why fuel pressure is regulated to the fuel rail.... to be able to
"see" this part better... a short lecture on "pressure" and "vacuum".
A "perfect" vacuum is something like 30 inches of mercury at sea level....
to the automotive PCM, this would be zero PSI of MAP (Manifold Absolute
Pressure). Zero inches of vacuum (again, we'll stay at sea level), or
ambient air pressure would give a MAP reading of about 14.7 PSI. Remember
this difference and read on...
And we need to touch on the fuel pressure 'spec' as well.... This is
written
with the engineers "assuming" that the reader has some previous knowledge.
Let's say the spec is 35 to 45PSI - what is meant is 35 PSI at no engine
load and 45WITH engine load....
At idle, we expect our car to have 17ish inches of intake manifold vacuum
(might be closer to 20 at sea level, but I live and work at about 2000
feet
above that and I'm used to 15~17 because of that). At this point, we'd
expect to see a MAP reading of 6 or 7 PSI. Knowing that our 'spec' for
this
example is 35 PSI at idle, we can do a bit of quick math... The pressure
on
one side of the injector nozzle is 35 PSI.... the pressure on the other
side
is about 6.5 PSI.... the pressure drop across the injector nozzle is going
to be about 29 PSI.
Let's get driving down the road... and let's get into the pedal a bit...
First thing we notice is that intake manifold has fallen to near zero
inches
(meaning our MAP has RISEN to near 14.7 PSI)... and we see fuel pressure
has
risen to 45ish PSI.. Some quick math and we see that the pressure drop
across the injector nozzleis in the neighbourhood of 30 PSI... Probably
more
than you wanted, but interesting stuff all the same...
While most fuel pump failures manifest themselves with "don't work period"
as the first symptom we see.... our driving styles and conditions can and
will hide signs of a failing pump from us. Corroded wiring connections,
corroded wiring, internal problems with the pump motor or mechanism,
restrictions in the fuel lines.... any of these can reduce or restrict the
fuel volume and pressure available at the fuel rail. Corroded wiring or
connections will affect the wires ability to flow enough current for the
pump motor to produce enough torque to build fuel pressure.
For a no start concern... we would check fuel pressure WHILE CRANKING THE
MOTOR. For a driving concern, we would check fuel pressure on the road
WHILE
THE CONCERN IS HAPPENING.
The two main things driving DIY auto repairs - the owner doesn't currently
have the finances to have the car taken to a shop or the owner feels pride
in being able to say "I fixed that!". (Or, sometimes, both). Either way,
I've seen a lot of money wasted on parts that weren't needed... enough
money
to purchase some of the tools that would have helped in diagnosing the
concern (and even might have left some cash in the pocket)... tools that
might pay for themselves more than once.
HTH.
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