Air/Fuel ratio gauge & Oxygen sensor

["Chip Mutza" <alfaracer75@domain.elided>]

Recently my 164S engine has been knocking quite a bit when under load. I
always use Premium (91 octane) fuel, and the knocking seems to ALMOST go
away completely if I use an octane boosting fuel additive. The knocking
seems worse with my AR Ricambi PROM chip installed, but it still exists with
the stock chip. My next step will be to check the cam timing, to make sure
the timing belt hasn't slipped.

Out of curiosity, I hooked an AutoMeter air/fuel ratio. I'm not sure exactly
how to interpret the readings of the gauge. My car seems to be reacting
correctly... at 80% throttle or more it reads rich, as the ECU ignores the
O2 sensor and relies on pre-set fuel maps. On deceleration it reads full
lean, like it's supposed to. All of the other times (0-80% throttle) it
bounces back & forth from rich to lean. According to the instructions that
came with the gauge, it should bounce back & forth many times per second...
I think my sensor is pretty worn out, as it cycles relatively slowly (maybe
2 times per second).

How am I supposed to know if the mixture is correct with the reading
bouncing back & forth like this?? I have also noticed that when it bounces
from lean to rich, it reads ALL the way to lean... and then only slightly
rich (two out of eight LEDs). The way I understand it, the reason the gauge
bounces like this is because the sensor is constantly making adjustments to
the mixture. So if the gauge is diving far into the lean area then the
engine is running lean, and the O2 sensor is attempting to correct it.
However, I can hear the knocking sometimes when at full throttle (under
load, i.e. uphill) and the gauge is indicating a rich mixture (normal). I am
assuming the knocking is not caused by a lean mixture.

So... I'm stumped. I'm SURE this is not good for the engine!! Any help would
be greatly appreciated!!!

-Chip Mutza
 San Diego, CA
 '93 164S
 '89 Milano (race project)
 http://www.fastalfas.com

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