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

[Ingo Rautenberg]

I tend to agree with Steve that the more likely culprit is a broken
thermostat.  I had replaced mine about a year and a half ago, but I have a
feeling this one's gone bad as well.  Any time you have a prolonged warmup
cycle, the ECU compensates to the detriment of fuel mileage.

Ingo

----- Original Message ----- 
From: "Buchholz, Steven" <Steven.Buchholz at kla-tencor.com>
To: <v8 at audifans.com>
Sent: Friday, April 29, 2005 7:12 PM
Subject: [V8] RE: cold weather killing gas mileage (AHA!!!)


The cover on the relay box has a listing of the fuse and relay
definitions ... use that as your guide.



Notice that I did not say anything about ambient temperature affecting
fuel economy ... I agree that the mileage the car gets is dependent upon
ambient temp.  I was specifically responding to the comment that the
failure of the OXS heater was the cause for the reduction in fuel
economy ... I'm pretty firm in that understanding ... any sign of change
in fuel economy dependent upon the ambient temperature is not due to the
OXS heater circuit not working.  I'd check the thermostat for sure ...
if the thermostat is staying open and the weather is cold you can have a
situation where the ECU deliberately richens the mixture because the
engine is cold!



Steve B

San Jose, CA (USA)

________________________________

Well, the microfiche that I have says it is relay J208; which is right?
(haven't been out to look yet.)  I know the fuse is good, but there
might not be voltage.  Gotta check that as well.

As for the ambient air temp affecting the gas mileage when the heater
wires don't work, trust me, it does.  It's a good mile per gallon
between 30 and 60 degrees, at least.  I don't know how, but it does.



**********

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 ...

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