[V6-12v] odd engine light problem?

[George Tur]

On Tuesday 20 June 2006 18:37, Rob wrote:
> Ok troubleshooting my intermittent check engine light in my 95 A6Q sedan.
> Basically I start the car and it drives fine and at some point during the
> drive the check engine light comes on.  I never see a change in performance
> and the car runs great.  Fuel mileage is about 23-24 mpg hwy.  I pulled the
> codes using SMJ's site, and here are most of them, I think I got them all,
> I'll double check tomorrow when I drive it back to work and the light will
> come on at somepoint during the drive.
>
***************
Had similar symptoms on the wife's car a few years ago and it was the egr 
system that was causing the problem. Around that same time there were a 
number of postings here about fixing that problem. One was by Nathan White 
and another was by Tom Christansen. The following is an excerpt from Tom's 
writeup on the egr fix. In the case of my wife's car I didn't clean the egr 
valve I replaced it. 

"Here's what I did:
First I followed Nathan White's excellent article on how to clean the
EGR passage:
http://www.mnsi.net/~natwhite/Audi/egr.htm
Summary: Remove large air hose and intake plenum. Remove misc devices to
gain access to throttle body. Remove throttle body. Remove EGR valve.
Clean EGR passage in throttle body. Clean EGR valve. Assembly is reverse
of removal. Remember to replace the throttle body gasket and EGR gasket.
The gaskets are about $8 total at the stealership.
Nathan suggests that you use a 7/8 inch flare nut wrench to loosen the
flare nut on the EGR exhaust tube. I could not for the life of me get
that flare nut wrench in there. I suggest using a 7/8" crowsfoot (3/8"
drive), 3/8" extender 8~10" long, ratchet wrench for the job.
Now that you've got everything disassembled, you might as well take the
IAC (Idle Air Control) valve - formerly known as ISV (Idle Stabilizer
Valve) - off for a good cleaning.
The appropriate torques (according to Bentley) are:
Throttle body bolts: 20 Nm
EGR valve flare nut: 27 Nm
EGR bolts: 10 Nm
IAC valve nuts: 10 Nm

After all that work I thought I was home free. But after about 15
minutes of driving that pesky amber light was laughing in my face
again!! ARGHHH!!!

Oh, well. A trip to Sears materialized one Miti-Vac vacuum pump tool. I
hooked it up to the brown EGR hose by the EGR purge valve (Audi
terminology for EGR solenoid valve). Applied vacuum and listened for
change in engine RPM. Idle RPM did indeed change as I applied vacuum, so
the EGR valve was working. With an ohmmeter I also verified that the EGR
temperature sensor was working (resistance decreased as temp was
increased by activating the EGR valve). With the engine off, I applied
about 85 kPa of vacuum. Waited for a few minutes to check for vacuum
leaks in the EGR valve. After a few minutes the vacuum was still 85 kPa.
So short of wiring faults, I concluded that the culprit was the EGR
purge valve. 

Now that I had to place an order with The Parts Connection
(http://www.thepartsconnection.com/) anyway, I decided to change
ignition wires and replace some vacuum hose bits.

The vacuum hose size is 3.5 mm ID for most of the hoses. I suspect that
the small hose bits are 4 mm ID as the 3.5 mm was pretty hard to make
fit on the hose fittings. But with a little dish soap the hose fit
snugly on the fittings... I bought one meter (3.3 ft) of hose, replaced
most of the hoses, and still have about a foot left of it.

The part number for the purge valve is: 054 906 267. It's about $90.
It's really easy to replace. It's located on the airbox underneath the
MAF sensor. Remove the large, flexible air hose between the MAF sensor
and the intake plenum. Un-clip the lid of the airbox just as if you were
changing the air filter. Bend the whole deal backwards (away from you).
The purge valve is a brown gizmo with an electrical connector and four
vacuum hoses to it. It's held onto the airbox with one Philips #2 screw.
Just pull the hoses off, change the valve out, replace the hose bits,
and put everything back together again."