[V6-12v] More little problems

[James4ihl at aol.com]

In a message dated 30/11/03 11:43:15 pm, touring_fan at msn.com writes:


> I was going home, going about 70 mph in a 65 mph on an interstate.  All
> of the sudden, there seemed to be some sensor problem.  While I held the
> pedal down, it would not go faster than 70 and it started to slow down.
> The engine seemed to be working because I can slow down to 60, then
> accelerate back and hit the limit again.  I pulled over and turned the
> engine off and let it sit for a few minutes.  I started it up and
> continued on and the problem didn't happen again for the 20 remaining
> miles on my trip.  This happened again the next day going 50 in a 45.
> In both instances, the rest of car was working fine.  I'm thinking that
> it's one or more O2 sensors messing up, sending false signals to the ECU
> and therefore limiting fuel delivery.  I think one or both catalytic
> converters may be damaged thanks to the local roads and if I'm right,
> the O2 sensors are in the exhaust near the cat's, the O2 sensors may
> have gone bad.  Anyone BTDT?  I'll drive it to the local Audi dealership
> to have them fix it and get it inspected in 2 weeks and thought I should
> get any insight I can.  Thanks, Nathan
> 

May be the O2 sensors, but there are a lot of other things that could cause 
that type of problem too -   no full throttle enrichment (caused by a number of 
things, e.g. Hall sensor wiring problem), loss of power due to a dodgy 
throttle valve pot or wiring to it, lack of signal from one of the knock sensors can 
cause loss of power or abrupt loss of power (more like a misfire), a faulty 
Mass Air Flow meter can also cause very similar symptoms... in fact one of the 
last things I'd attribute to that fault is a lambda probe - the only effect a 
dodgy probe should have is poor emmissions - if one goes down completely, the 
ECU should just operate in open loop (it uses all the other sensors and stops 
using the 02 sensors in feedback loop to control emmissions). Some of the 
symptoms of the knock sensor control limit being reached are:

- slight loss of power
- maximum speed too low (!)
- rough idling

You may have all or only some/one of these. It can be caused by faulty 
screening in the wiring to the sensor, a loose sensor (should be 20Nm), poor grade 
fuel (under 95 RON), a malfunctioning knock sensor, or the knock control unit 
inside the ECU could be shot.

Before you replace the 02 sensors, I'd try to have it hooked up to a VAG-COM 
and see if there are any fault codes present, or if, like you say, you're 
taking it to your Audi dealership, get them to put it on a diagnostic computer 
first.

Good luck!
James