[V8] cold weather killing gas mileage (AHA!!!)

[Buchholz, Steven]

I looked at the 1991 wiring diagram last night.  The heater is powered
by what appears to be two relays in parallel ... one of them being the
fuel pump relay (J17 - Position 10).  There appears to be a 5A fuse S5
in line with the heater, you should be able to measure voltage there to
see if it is a supply problem.  

This morning I pulled up the soft copy I have ... looks like the pre-91
cars only used the single fuel pump relay.  Funny thing is I don't see
any way that the ECU can determine the status of the OXS heater from the
wiring ...

I also wanted to comment on the statement regarding the outside
temperature influencing the functionality of the OXS.  I am very
skeptical that if you had a non-functional OXS heater that the ambient
temp would influence the functionality of the OXS proper.  The sensor
operates at temps much higher than ambient.  I would expect that if the
heater for the OXS were not functioning that the OXS would likely get
warmed up sufficiently to operate properly while the engine is under
load, but would cool down and malfunction when the engine is idling.  Of
course the ECU trusts the OXS signal at all times, so I'd expect there
to be problems with the idle quality and fuel consumption at idle in
this case.  

Steve B
San Jose, CA (USA)
> 
> I am pretty sure (but I have never actually traced it out) that the
12v to the
> heater comes directly from the ECU. There is no mechanical relay to
replace -
> rather there is an electronic switch that may be replaceable only if
you could
> source the part (maybe 50/50 chance it is an off the shelf part) and
you were
> capable if identifing the part and actually repairing the ECU. Not a
simple
> task.
> If I get time I will try to trace out the heater wires and figure this
out for
> sure.
> 
> Dave
> 
> 
> Quoting quickaudi at aol.com:
> 
> > Guys, it never ceases to amaze me how many good uses a VAG-COM (or
even
> > better, an actual VAG computer that your new buddy who works at a
Volkswagen
> > dealership uses to run codes on your car) actually has.  We pulled
the
> codes,
> > and the only one that came up was 00537 - Oxygen sensor control -
Adaptation
> > limit exceeded.  Well, they obviously don't have the number
reference for a
> > 1990 Audi V8q at a Volkswagen dealership, so we ran the search for a
car
> with
> > similar programming.  00537 came up as Heated Oxygen Sensor (HO2S)
heater
> > circuit malfunctioning.  It is telling me to check the voltage from
the
> relay
> > and to check the wiring from the relay to the heater wires.
> > This explains so much.  Basically, if we understand this code right,
the
> > oxygen sensor heater wires aren't getting either any voltage or full
> voltage.
> >  When we restarted, we saw that it said intermittent, which really
leads me
> > to believe that I have a relay that isn't working quite right, as
opposed to
> > a bad wire, but I won't rule out either.  This is why my gas mileage
was
> > worse in the winter and better in the summer.  The heat from the day
would
> > actually help warm up the O2 sensor and therefore help it to read
properly.
> > I've been running the car rich for quite a while, which kinda gets
me.  I
> > remember hooking the computer up to it a couple of years ago, and we
saw the
> > code, thought it was the 02 sensor, and erased it.  I later replaced
the
> > sensor, thinking it to be fixed.  This goes to show: run your codes!
At any
> > rate, if anyone has any ideas about which relay to check, what to
look for,
> > etc., please let me know.  Maybe this will help some others on the
list as
> > well.