[urq] What causes knock ?

[fdekat]

> ------------------------------
> 
> Message: 7
> Date: Thu, 11 May 2006 21:13:09 -0400
> From: "Louis-Alain Richard" <laraa at sympatico.ca>
> Subject: [urq] What causes knock ?
> To: "'Urq List'" <urq at audifans.com>
> Message-ID: <002b01c67561$3b5c9ac0$6401a8c0 at dell8200>
> Content-Type: text/plain;	charset="us-ascii"
> 
> My car developed some strange sounds, and one is sounding a lot like
> knock. It happens only under boost, and it is sporadic. But it scares
> me, specially after what happened to Dennis...

> And my title question remains: what causes knock ?
> 
> Louis-Alain


Hello  Louis-Alain:

I've had audible 'pinging' twice on my MAc 02'd MC motor in the urq.

First time I noticed it was when I got all the intake leaks sorted out, and
was *finally* getting some boost.

The cause was the ECU not seeing any intake manifold pressures (or vaccum).
Audi has installed a 'water separator' on the small vaccum line from the
manifold to the ECU. It looks like a small gas line filer, with no innards,
and I found mine up under the glovebox, near the ECU.  

However, it had cracked from old age, and there was no way any
vaccum/pressure signal was going to make it to the ECU!

I replaced the 'filter', and found out all of a sudden, it didn't pull as
well (as the ECU was now retarding the timing, as it should have been doing,
all along!)



The next summer, I heard what I though was 'pinging' again. But on
investigation, could now find no leaks. Which lead me to surmise that the ECU
pressure transduce had taken a dump.  Sure enough, when I measured the output
from the transducer, it sat a '0' volts, no matter what happended.   So
timing was being set to 'full vaccum' values, rater than the '1 bar'
atmospheric that the vaccum leak previously would put it to.

No surprise that my head gasket failed in there somewhere too!


My workaround for the failed pressure transducer was to rig up a switch that
fed a 'full boost' signal to the ECU when pressed.  At least when I pressed
the switch, the ECU set timing to boost levels, preventing more pinging.

However, as that was a bit of a hokey arrangement, and no MAC 02 pressure
transducers were going to appear, I swapped over to full MAC 11 ecu and
harness and ancilary devices.  May as well, as the motor was already a MC
anyway..!

So, short form: 
Make sure the ECU IS seeing manifold signal, and that the ECU pressure
transducer is working!

Regards,

=Frank de Kat=