[V8] MPG - again

[Jason Wilkerson]

Tony, Thanks for the detailed response - I'm going to try to answer your
diagnostic questions below - my answers have the hyphens.

With the bad O2 the car runs, but bad MPG (probably rich). 
  - As far as I can tell the bad sensor puts out nothing - so the ECU sees
no change in signal, thinks it is disconnected and goes to closed loop,
hence no try for correcting a rich or lean condition - just goes to full
rich to 'save the motor' - hence bad MPG

On the good o2 the car dies at idle, and puts out a "bad O2 code" but you
checked the output voltage and it seems fine. 
  - Not true, the 'good' o2 shows the other code, that it is out of it's
'correcting' range - it could possibly be a bad o2, but I checked the
output at the sensor (put a meter on the sensor wire and one to engine
ground) it was sweeping from about .3 to .75v - seemed correct to me.
 
BTW, what year is this car?
  - 1990

When you disconnected/reconnected the O2, did you by chance bump the 
engine speed connectors? 
  - I don't think so - if I disconnected either the car would be hard or
impossible to start.

How could it not be the O2, as the new one kills the car, but how could 
it be with a good signal?
  - When I say good signal, I mean I think it is doing it's job - my
understanding is this: The o2 finds a rich or lean condition and changes
other parameters (timing, injector dwell, etc.) whether there is a problem
in the system or not the ecu trys to correct it.  If you have a problem,
say a vacuum leak - it doesn't know the proper thing to do because the MAF
says that you are receiving this volume of air, but the engine is actually
seeing more, so that's why I think the engine dies.  When it dies it will
run fine for awhile, then at idle it will start to surge (correcting
excessively), gradually loosing rpm and then die.  You can make it run if
you 'feather' the gas, then it may run for awhile, then act up again. 
Whew, long answer, huh.

In reference to the ground they are talking about, that is the basis 
for the 0-1V output. If the ground is bad, that signal may read too high.
IOW, ground is 0V. If the ground is bad, I've seen as high as 6V relative
(with the battery negative as the baseline). So, the computer would see
7V! This is one of the reasons for the 4-wire modification, although it
should certainly not be necessary to get the car running correctly. The
worst I've seen is terrible surging during acceleration in a V8.  
  - I'm going to back-probe the wires at the ecu itself and try to see
what it sees, I may also stick a jumper on the new sensor base and take it
to the block - just to see what happens.

Hope this helps,
             Jason



 
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