[V6-12v] Low Gas Mileage

[Tom Christiansen]

Nathan,

If you get the EGR code, clean the EGR passage. Check out Nathan White's 
procedure for doing this. <http://www.mnsi.net/~natwhite/Audi/egr.htm>. 
Nathan White uses a 7/8" flare nut wrench for the EGR tube connection. I 
couldn't get a flare nut wrench on the damn thing, so I ended up spending 
$$ on a set of crows feet (looks like the open end part of a wrench with a 
3/8" square for a rachet where the handle should be). I bought a metric set 
and I think I ended up using a 22 mm, but I'm not 100 % sure of that 
number. Anyhow... Once you get that nut loosened up the rest is easy. Soak 
it in penetrating oil the night before...

You'll need a piece of wire cable for cleaning the EGR passage. My bicycle 
still doesn't have a rear brake cable as a result of this procedure... ;-)

Remember to clear the trouble codes in the ECU once you're done with the 
procedure. There are several ways of clearing the codes. The one that works 
for me is to disconnect the battery. Wait a minute. Then reconnect it. Make 
sure you have the radio code before doing this, though.

Bad news was that this procedure did not solve the problem for me. If you 
after cleaning the ERG valve and passage still get the code, get yourself a 
mity-vac tool and apply vacuum to the EGR valve (with the engine running). 
If the RPM change as you open/close the EGR valve the EGR valve is working. 
If not - clean EGR passage, tube, valve. If the valve doesn't hold vacuum 
(the diaphragm is leaking) replace the valve.

If the EGR valve is working, the other options for causing an EGR trouble 
code are the EGR temp sensor or the control valve. The temp sensor is easy 
to test if you have an ohmmeter (or multimeter). The EGR temp sensor 
connector is found in the connector bank behind the intake plenum (or maybe 
it was the small bracket by the EGR valve. I think that was it). It's a 
really skinny wire with a two-pin connector on it. The sensor itself sits 
on the starboard, lower corner of the throttle body and cannot be seen from 
the top (at least I can't see it). When you open the EGR valve (apply 
vacuum) with the engine running, the resistance of the EGR temp sensor 
should change quite dramatically (by at least 1-2 order(s) of magnitude). I 
seem to recall 200 kohm (-ish) when the EGR valve was closed and 2 kohm 
with the valve open. If you don't see a change in resistance, the sensor is 
bust.

If the temp sensor and EGR valve work, then the final option is the control 
solenoid valve. It sits on the airbox upstream of the MAF sensor. Just 
follow the EGR vacuum hose/tubing and you'll find it. At this point you've 
hopefully verified that you don't have a leaky vacuum hose... Replace the 
solenoid valve.

If this doesn't clear the fault, you're probably looking at defective 
wiring to the ECU, defective ECU, etc.

Tom

At 07:20 AM 5/31/2005, you wrote:

>Well, I have a problem with my car. I was making a 100 mile trip and by a
>calculation with an estimation of the gas levels, I'm achieving about half
>of the gas mileage I should. Now, the EGR system still trips the check
>engine light, it was on practically the entire time during that 100 mile
>trip, I still need to get that fixed but I'm not sure that would cause the
>low mileage. I talked with my dad and he says we might want to change the
>spark plugs and/or wires but then he said we might want to just take it in
>to be tuned up. Does anyone know something more specific worth checking out
>to help narrow down the problem so I can say "I need this to be replaced but
>I don't have the tools" as opposed to "it doesn't work good, fix now". I've
>archived the email discussion about the EGR system from a few months ago
>when I brought it up before. The car is a 1994 90 with everyones' favorite
>12V V6. Any help you guys can provide will be a great help, thanks a lot,
>Nathan.
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