MAC11 help needed

Louis-Alain Richard laraa at
Mon Sep 1 20:28:02 PDT 2008

If they are OK, then maybe something is triggering the knock sensor ?
Something loose in the vicinity maybe ? One usual suspect is the vacuum pump
rod that is located in the head. I know an MC doesn't have one, it's a
normally aspirated engine gizmo, but you may want to check for rattling bits

Then, timing tables are dependent of numerous sensors : RPM, manifold
absolute pressure, intake air temperature, WOT and idle switches, and knock
sensor.  Did I forget something ?

You said the erratic behaviour appeared after a rainy session, then maybe
one sensor signal is impeached (corrosion, short, etc) and that signal is
only processed after 2500 rpm, causing the rough running. You just have to
find each sensor impact on timing retard to help you diagnose: guys with MC
knowledge should chime in here. Or guys with a Bentley for the MC...

Finally, a few years ago, I had a similar problem on an Iron Duke engine (GM
2.5 Litre OHV 4 cylinder for S-10/Sonoma). One mechanic took all of 2
minutes to point his finger at the distributor reluctor, the round magnet
that triggers the Hall effect sensor. It was originally press-fitted on the
dizzy shaft, but 10 years on, many small cracks had developed and the
reluctor was free to move a few degrees around the shaft, causing the
backfires. Replacement was the only solution.

Hope this helps,

1983 Quattro

> -----Message d'origine-----
I have tried different torques on the original and spare knock
> sensors with no change, but they are used, not new, but from decently
> running cars. 
> Keep your thoughts coming.
> Rick

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