350TPI Fuel Consump Problem

[Shannen Durphey]

Hi, Don.
I've gotta say, if you're having trouble with fuel mileage around town
and in traffic, and you are watching block learn values at 65-80 mph,
I sure don't want to get tangled up with tht traffic. ; )

Specs for fuel pressure in my manual are 40.5 - 47 psi @ key on/engine
off, 3 - 10 lbs less with engine running.  With your lightweight car
and stock engine, I wouldn't blame the trouble on fuel pressure
without checking for other problems first.  Disconnecting the vacuum
line to the regulator should bring fuel pressure back up around
initial reading.  Pinching the fuel hose on the return line should
yield pressure around 70-80 psi or more with the engine running.  Some
'Vette systems that cannot make this pressure are prone to hard starts
and hesitation.  You should attach a fuel pressure gauge to the car
while driving if it's possible.  I find it very hard to duplicate road
conditions while the car's sitting still in the shop.

Be sure you keep eyes/ears open.  Black, sooty buildup around the
tailpipes, carbon fouled plugs, low vacuum readings are all signs that
there may be other problems.  An ecm will do bad things if the info
it's receiving is bad.  'Specially with O2 sensor readings.  (Hello,
Bob V.)  Be sure to check the basics, and sometimes check them again. 
Disconnecting the O2 is a very crude test to isolate the electronics
from fuel/iginition/other systems.  If the vehicle will run correctly
without the O2 sensor, it's *probably* not a fuel or ignition
problem.  Remember to collect as much data as you can, and change one
thing at a time.  Sometimes the problem is really two or 3 smaller
ones working against you.  One leaking injector can cause one cylinder
to misfire intermittently, which can give an occasional lean reading,
which can cause the ecm to add fuel, which will richen all cylinders
including the one that's misfiring, which will misfire more
often.....  It helps to monitor as much as you can and keep your wits
about you.  

When you are snapping the throttle, the o2 volts will drop rapidly,
rise rapidly, level out (unless you're going into open loop).  Speed
is relative here, but the lean/rich snap should be pretty darn quick. 
Slower can be an indication of insufficient fuel.  Letting off the gas
from higher rpm with the car stationary will cause rich readings with
most stock calibrations.

I believe a service manual is an essential tool for this type of
work.  And not an Auto Zone supplied Haines manual, either.  Don't
consider the scantool readings to be the final word on what's
happening.  There are many other issues that come into play in
driveability.  Bad grounds, corroded terminals, slightly skewed
sensors, electrical noise are all possible culprits.  You don't have
to be great to fix a driveability problem, but if you're not observant
and thorough, you'll waste bunches of time and energy.  

Just trying to get ya in the right frame of mind for diagnostics, and
help ya get to the problem as quickly as possible. : )

Shannen


Don Burns wrote:
> 
> I posted to the DIY list a few weeks back about my mileage problem and then
> left town for a while. Am surprised that someone didn't bite my head off
> since the question really belongs on the GM list. Anyway many listers are
> on both so you probably saw this before.
> 
> The car is an 86 Fiero with a crate 350TPI and 5-speed. Its closest
> equivalent is probably a '90 350TPI in a F-body. Problem is the fuel
> consumption sucks, expecially in traffic: 12MPG. On the road it does 20+.
> 
> At idle, integrator readings range from 130 to 140. Block learn seems to be
> pinned at 160 (this is the mac, right?) Earlier somebody asked (I think
> Shannen) about fuel pressure. I think I said 40 or 41 lbs, but I lied. So
> much for my lack of thorough note taking (Bruce, you were right). The fuel
> pump is seeing about 0.6 vplts lower than battery (my cutoff switch wiring
> is probably responsible).
> 
> So today I started again from scratch. With fuel line pressured up and
> engine not running pressure is about 42 lb. (and stays there... no leaks).
> Started engine and pressure drops to 36 lbs and stays there. I think this
> is much too low.
> 
> Took the car out on I75 this afternoon and found following at 65 to 80mph:
> 1. Integrator drops nicely into mid 120s.
> 2. Block learn starts dropping and even gets into 140s... nice sign. Also
> found that when punching the gas the block learn quickly drops into the 120
> to 130 range and then in a second or two begins to rise into 150s. I assume
> this may be the ECM detecting a lean condition and then eventually trying
> to compensate for it.
> 
> Getting back to the fuel pump, is my FI system starving because of lack of
> high enough pressure and then trying to compensate?? Will appreciate some
> feedback on this. Another question: If the FI is starving what is a quick
> way to determine if its the fuel pump or prseesure regulator opening early??
> 
> Still have to answer Shannen's question about running with O2 disconnected.
> Need to jack car with floor jack to get to connector but the sciatic nerve
> suggested today was not the day to do this.
> 
> Thanks for the help!
> 
> Don
> 
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